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I'm not really sure it can. You can prolong the time before it dies by trying your best to keep it underground-ish. I think reddit's downfall started with the death of Digg. Digg had a huge user base that left for reddit almost all at once. At that point reddit's community shot through the roof and then kept growing. It's not Digg's huge user base that killed reddit, but rather Digg had already started the eternal September (aka full of stupid kids) and when Digg dumped it's user base on reddit, it took the kids with it.
Basically try your best to keep this site relatively small. I don't think we should do anything drastic like making the site 18, mainly because it would have the same effect of making porn sites 18. Plus, don't forget that not everyone under 18 is a dumbass, I'm 17 and have been around a pretty long time.