It's not so much age as it is maturity. I know plenty of people around my age (15) who are actually more serious than I am in real life. It's also like saying that older people can't be immature (which is a bit... strict?).
And, frankly, it's difficult to "choose" what "style" you use. Once you start writing in a certain way (and remember that we do communicate by writing, not talking), it's difficult to stop.
Sorry about the off-topic post....
But I just must brag....
http://forum.toribash.com/showpost.p...&postcount=845