I used to be really childish on forums because I thought it was "cute" and "cool" and more people would like members who acted that way. You know, using smilies after almost every sentence, and posting random words that are usually said in anime, and written in manga.
Though I did maintain a surprisingly high maturity level for my age during that time at school, if nowhere else. I am proud to say I am even more mature at this moment after being a few years older than before.
All in all, I have been pointed out as being mature for my age by my teachers, mainly because I acted like the shy kid in class instead of the class clown. I could strangely have a normal conversation with any of my teachers as well, even as a freshman.
I guess maturity can mean that you will not start giggling when you hear words that used to be funny in grade school in your anatomy class, and you know when to goof off with your friends, but you know how to act perfectly calm in a classroom. There is really more to it than that, but my brain is not working properly right now on account of the lack of sleep.