Sweet! As for defining the age, shouldn't be much of a problem, once you see my post. 
Note to the community: This is one giant Devil's Advocate on my part, mainly only taking place after some out-of-forum discussion between the two of us... I don't plan on starting some massive suicide cult. 
I want to start off with a small side note. I believe the best way to calculate the best course of action is to evaluate the viewpoint which most effectively protects the most lives. That being said, we must remember one thing: Humans are not fully self-sufficient. We rely on forests for oxygen, a stable biosphere for food, and various animal species existing in order to sustain biospheres. Even outside our planet, we require the sun to provide sunlight. Thus, as much as human life is valuable, the world around us is important to sustain that human life.
There's two key arguments here:
1: Our impact upon the universe.
Humans today are on the verge of achieving a number of technological breakthroughs. Now, we normally see many of these as generally good things. However, there's a great risk with each coming technology. Humanity has so many different technologies expanding, all at the same time. With modern societies, this is inevitable, as the drive for progress creates ever-expanding growth. That being said, these always create the possibility of unintended consequences.
A: We're on the verge of a massive breakthrough in nanotechnology. Nanotechnology, when fully materialized, has the possibility to revolutionize the way we live, by manipulating matter at the atomic level. This is pretty awesome, and could have huge implications for every aspect of the way we live. The very profitability of the technology, even being seen today, would indicate that this makes nanotechnology's development inevitable. Even national governments, including China and the United States, are in races to see which nation will be the first to garner the benefits from fully maturing nanotechnology. And no, this is not something coming out in 50 years.... nanotechnology is being matured today. Authors such as Ray Kurzweil have said that fully matured nanotechnology will be a reality within 10-15 years. The future is here.
That being said, many scientists have highlighted a major fear in nanotechnology's development: the "gray goo" problem. In order to most efficiently produce nanomachines, most machines would need to be able to self-replicate, taking matter from outside the environment to produce what is needed. Using external machinery to create final outputs of nanotechnology would either be inefficient, requiring human inputs when none are needed, or would be self-regulated, resulting in the same system, in effect.
So most nanotechnology will take the form of self-replicating machines. Okay. Except that it only takes one glitch, one failed experiment, or one deliberate reprogramming of a single machine in order to turn a productive nanofactory into a global disaster. Imagine if just one of those self-replicating machines were to be programmed to produce copies of itself using materials that could be easily obtained from the environment, such as oxygen or carbon? Or imagine if a safeguard to normally tell a nanomacine to stop reproducing... suddenly stopped working? Either of these could happen quite easily, by either accident or design.
And the results would be disastrous. Eric Drexler, a physicist who first proposed this theory, laid the calculation out quite plainly. If a single nanobot could produce a copy of itself once every half hour (a fairly conservative estimate), the mass of nanobots would weigh the size of the Earth within just 2 days, destroying the planet in the process. In addition, because matter travels between spacial bodies, there is a risk that these machines could spread even beyond the Earth, reaching other asteroids, planets, and even outside our solar system.
B: I know this website probably isn't the most legitimate of websites, but it is fairly effective in explaining the argument.
http://www.exitmundi.nl/quantum.htm
"It will be over before anyone can say `sorry'. In a laboratory somewhere, someone tries to get hold of a weird and completely new, exotic type of energy. But boy, the experiment goes out of hand. Suddenly, there's a BIG explosion. And then there's nothing -- our planet, the sun, all planets in our solar system and even some stars surrounding our solar system have been blown to smithereens.
And explaining what went wrong isn't even simple. We're talking quantum physics here: the physics of the vanishingly small building blocks that make up all matter in the Universe.
In quantum physics, everything is totally different from daily life. Quantum particles can be in two places at the same time, and can behave both like waves and particles. In fact, when you hear a quantum physicist say `particles', don't think of little, round balls. Quantum `particles' are better compared with tones of music: they're definitely there, but you can't see them or catch them.
One of the most mind-boggling properties of quantum particles is that they come into existence out of nowhere. Suck every molecule of air out of a bottle, making it completely vacuum -- and quantum particles will still be there. They pop up in pairs out of nowhere. And within a tiny fraction of a second, they merge together and -- zzzip! -- they're gone.
It is precisely this odd `quantum vacuum' that may one day open the door to a very new source of energy. Suppose you're able to snatch some of those out-of-nowhere particles away. Admittedly, you'll have to be REALLY fast. But if you do succeed, you'll have harvested particles out of nowhere. And since matter and energy are basically the same stuff (according to Einstein's E=mc2), you'll have energy out of nowhere!
The advantages would be unimaginable. Here's an energy source that never runs out, is everywhere around, is extremely cheap, and causes no pollution whatsoever.
But then again, there is a small, but alarming risk. There may be simply energy too much. Mining the quantum vacuum might bring about an unstoppable chain reaction, releasing an ever increasing amount of energy. In fact, no-one knows how much energy will be released: calculations done by physicists give answers anywhere between zero and infinity.
Obviously, too much energy would mean trouble. The explosion could be huge enough to blow apart our entire solar system and everything around it. And of course, infinite energy would bring about infinite destruction, bombing not just a handful of stars, but everything in the entire Universe.
Gladly, no present-day scientist is capable of mining the quantum vacuum. On the other hand: one day, there will be. And that day may arrive sooner than you think: some estimate around 2020 science will be ready. Let's hope physicists finally have their calculations straightened out by then."
There's not much I need to add to this, at least for now...
C: Just call this scenario "Rise of the machines."
http://www.wired.com/wired/archive/8.04/joy.html
By Bill Joy, co-founder of Sun Microsystems and a computer scientist. He's very well-known for this specific piece, "Why the future doesn't need us."
"First let us postulate that the computer scientists succeed in developing intelligent machines that can do all things better than human beings can do them. In that case presumably all work will be done by vast, highly organized systems of machines and no human effort will be necessary. Either of two cases might occur. The machines might be permitted to make all of their own decisions without human oversight, or else human control over the machines might be retained.
If the machines are permitted to make all their own decisions, we can't make any conjectures as to the results, because it is impossible to guess how such machines might behave. We only point out that the fate of the human race would be at the mercy of the machines. It might be argued that the human race would never be foolish enough to hand over all the power to the machines. But we are suggesting neither that the human race would voluntarily turn power over to the machines nor that the machines would willfully seize power. What we do suggest is that the human race might easily permit itself to drift into a position of such dependence on the machines that it would have no practical choice but to accept all of the machines' decisions. As society and the problems that face it become more and more complex and machines become more and more intelligent, people will let machines make more of their decisions for them, simply because machine-made decisions will bring better results than man-made ones. Eventually a stage may be reached at which the decisions necessary to keep the system running will be so complex that human beings will be incapable of making them intelligently. At that stage the machines will be in effective control. People won't be able to just turn the machines off, because they will be so dependent on them that turning them off would amount to suicide.
On the other hand it is possible that human control over the machines may be retained. In that case the average man may have control over certain private machines of his own, such as his car or his personal computer, but control over large systems of machines will be in the hands of a tiny elite - just as it is today, but with two differences. Due to improved techniques the elite will have greater control over the masses; and because human work will no longer be necessary the masses will be superfluous, a useless burden on the system. If the elite is ruthless they may simply decide to exterminate the mass of humanity. If they are humane they may use propaganda or other psychological or biological techniques to reduce the birth rate until the mass of humanity becomes extinct, leaving the world to the elite. Or, if the elite consists of soft-hearted liberals, they may decide to play the role of good shepherds to the rest of the human race. They will see to it that everyone's physical needs are satisfied, that all children are raised under psychologically hygienic conditions, that everyone has a wholesome hobby to keep him busy, and that anyone who may become dissatisfied undergoes "treatment" to cure his "problem." Of course, life will be so purposeless that people will have to be biologically or psychologically engineered either to remove their need for the power process or make them "sublimate" their drive for power into some harmless hobby. These engineered human beings may be happy in such a society, but they will most certainly not be free. They will have been reduced to the status of domestic animals.1 "
Oh, sorry, I forgot to mention: This specific passage was a quotation from the writings of Ted Kaczinsky, the Unabomber responsible for various bombings at research facilities. While his actions weren't necessarily justified, his argument still stands. Assuming computer technology is advanced to the point where it meets, and exceeds, human capabilities, humans would no longer have the monopoly on intelligence. In effect, we would be relegated out of existence. Not necessarily through some giant war, like you've seen in movies. But either way, computer technology would be the future, and life would be obsolete.
2: Our impact upon our own biosphere.
As humans moved outside their stone-age civilizations, they transformed into what modern ecologists call an invasive species. We traveled to biospheres that were, at the time, not adjusted to accommodate humans. With each advancement in modernization, we step closer toward pushing out various biospheres. A few examples:
Global agriculture removes huge swaths of land from natural use. In many cases, this includes land which was vastly important for the world. The best example here is the Amazon Rain forest, currently being destroyed to make room for cattle ranching and farming to feed millions of people.
Biotechnology-based plants have been extremely strong in the capability to compete with other plants. Rice seeds that have been allowed to escape containment have easily been able to outcompete most crops. if given the capability to reproduce freely, such plants could easily become invasive species across continents, destroying any ecological balance on the planet and destroying the global food supply.
Resource extraction is devastating to regions of the planet. Mining interests to this day fight against environmentalists interested in protecting key regions. With just one favorable election, ecologically protected regions could be removed from protection status, mined, and set to points where the ecological properties are modified enough to dejustify protecting the region, losing key biospheres.
Monocropping is one of the most dangerous farming techniques, and it's extremely common in the developed world. By only using one crop in farming regions, the land being farmed is degraded to the point where it loses its minerals, leaving the land to become unable to sustain most life.
Every form of energy production currently produced, with the exception of solar energy and possibly geothermal energy, have some damage upon the environment, whether it's carbon monoxide emissions, thermal pollution, the risk of massive spills, or any other form of pollution. Each one has devastating effects upon the environment.
Industrial-level resource extraction doesn't factor in the needs to balance the environment. Overfishing and whaling risk throwing ocean biospheres into chaos, in particular.
Why do I list all these problems? Because nature is a fragile being. It depends on a large level of genetic diversity in order to survive. If enough species that fill a particular niche are removed, the entire tapestry unfolds.
Let me give you two examples of this:
1: The bees. Honeybees are probably the #1 best example of a keystone species. They are needed globally in order to pollinate a number of plants. Today, however, they are starting to die out. Nobody's exactly sure why. A good portion of theories present some form of human interference as the cause, as Colony Collapse Syndrome has only been widespread recently.
2: Whales.
http://www.biologicaldiversity.org/species/birds/spectacled_eider/worldcatch.html
The story is insane. 100,000 otters inhabited the Aleutian islands. Then five baleen whales came into the region. Within 5 years, the numbers dropped to 6,000 otters. All because of one extremely tiny instance of an invasive species moving into an environment.
I'll avoid the global warming debate with you. I'm more familiar with the science of nanotechnology and artificial intelligence than climate change.
Only typing this because I woke up in the middle of the night.
By the way, the Stone Age isn't on the list!