I've been posting on various different boards for years. Every time I look back on the old posts I made back then, I get a tinge of embarrassment, because it's so easy to tell from the language I used, the way I structured and punctuated my sentences (ie: poorly) and the way that I used to talk to people in general conversation, that I was young.
I would make jokes that an adult simply wouldn't find funny, I would try to form opinions on issues (because I saw all the other, adult posters around me doing the same) that I knew nothing about, and ended up saying a lot of stupid and arrogant things.
I had no idea I was coming across in such a childish way at the time, but looking back I can tell in a second that I could only have been around 13 when I wrote those comments, and if I can tell now, then those other posters could tell at the time. When I wrote all of those posts I was convinced that I was mature for my age, and couldn't see the problem with what I had written, it's only after a lot of growing up that I've realized how dumb I was.
It doesn't matter how mature you think you or your friends are, because if you're still at a young age, then you are still immature in comparison to an adult, and people WILL be able to tell how old you are from your posts, even if you see nothing wrong with them. It's not a bad thing really, age doesn't make what you say any less valid, and immaturity is unavoidable when you are young, but people can still tell, and will try to use it against you.