I'm a bit ambivalent about a minimum age requirement. I can see the point that it sends a message that a certain degree of maturity is needed, in for example ability to take criticism and what not, especially if you want to go on the forums. However, as has been before, people will inevitably lie about their age to get on the site, especially if they think they are great writers and want to showcase their work.
In that sense, I think an actual email confirmation system will do more to deter immature users. Even with age requirements, you'll only be able to find out if someone is actually decent enough to be on this site after they for example post a story and whine about the criticism. However, even if certain terrible people get banned, they can easily make a new account and put up part two of 'Sentient cat-thingies that explore each others' bodies', then get banned again, create a new account, and so on. There are of course ways to get around e-mail confirmations, but it does take a bit more work, and will probably deter the more (both literally and figuratively) immature trolls.
Furthermore, if the 'school mode' system ever gets implemented, that will also prevent the flood of mediocre games that happens once every while when a teacher decides to encourage their kids to write stuff online.
All in all, age requirements are a purely symbolic measure, that are easily circumvented. In my opinion, there are other, better ways to deal with immature users, without blanket banning kids.
And I do apologise if my rant was a bit incoherent, but I think I have a mild caffeine overdose at the moment from a day of coffee-fueled paper writing.