Maturity level and physical age are two different things.
It's the internet and we can all lie about our age. But can your posts tell others how old you are? Or does it say more about your maturity level. I think it's more the latter.
Game forums notorious for having kiddie posters. We call them kiddies, I suspect, not necessarily because they are kids but that they behave like kids. Bad rap for real kids, considering there are many kids and young forumers who are civilised, well mannered and thoughtful with their posts. Some kids don't know how to behave on the internet, but don't assume that they are all like that or that childish behavior is only restricted to children.
When we see a horrible post with bad grammer and spelling, we naturally assume that "a child did this". Not fair and not always true. Sometimes it's because the person is:
1) Dyslexic
2) Not from an English speaking country.
Such things are understandable.
It's not hard to distingush just-bad-English from just plain don't-care attitude and rudeness.
Anyone who scawls a post with barerly readable sentences and then tells you to "Fuck off" or "U R Gay" if you have a different opinion is definetly being rude. If confronted, and they say they don't know English, you can respond that they certainly know enough English to be as rude as a rude English speaking person.
The only way or rather, the best way to estimate someone's physical age is to see what they like to post most about: is it about school, work or angst? (Even so, it's not always right on.)
If someone tries too hard to convince you that he or she is thirty, that person is almost definetly NOT an adult in the physical sense. No adult would boast about reaching three zero, not like it's some kind of trophy at least. "I am old enough, I am thirty." as if that is suppose to make all of us suddenly accept his point of view. Well done, genius.
Don't assume a well thought-out post is necessarily made by an adult - this can sometimes lead to embarrassing emails and chat dialogues. Some young people, kids can be intelligent and insightful too.