The problem is that you could be targeted because of the age you post. That's what we're trying to get at here.
Some creep is attracted to how young you are, searches your posts on the forum and gets more personal info and maybe pics, joins the fb group and gets your real name since there's a list of who you are on fb and your avatar's name, they friend you in game, and it goes from there.
If posting your age on a forum makes you such an easy target, I'd hate to see what danger having a facebook profile puts you in.
Again it comes down to personal choice. Feel it's too personal to post your age? Don't post it. Some don't care, so there shouldn't be an issue with those who are inclined to share their age publicly. I think majority of those that posted their age publicly on here are old enough to do so, so I don't understand where this whole thing came from.
Ages people have posted so far: 14, 16, 17 almost 18, 19, 19, and 20 (myself). In my opinion, none too young to be forthcoming with their age.