After reading a bunch of forum posts at once, I notice that people are posting their age a lot, either in the body of a message or in their signature.
Although not as bad as posting your exact birthday, your age could be combined with other information about you to determine your identity. I think just your approximate age ("I'm 10" vs. "I just turned 10 yesterday") is low-risk, especially if you're not stupid enough to go meet someone in person based on online conversations. But it's not zero-risk, and it's a little risky for us, too, to have such information about you, especially if you are under 18.
I'm always curious about how old people are, so I understand the temptation. But, check with your parents before you post your age, at least. Thanks.