How would you enforce this? I can be 14 and say that I am 18. I don't think enforcing the age is feasible unless you have some users that are honest about their age. But as soon as others notice people are being banned (whatever action is taken against the minors), I believe they would lie about their age to prevent the same fate.
The difference between stupidity and genius is that genius has its limits.
When you take things for granted, the things you are granted, get taken.
Even a mosquito doesn't get a pat on the back until it starts to work.
Too many people miss the silver lining because they're expecting gold.
[BES 5.0.3 / GroupWise 2012 HP2]