Shafin's right. You don't have to know the persons age to be able to talk to him through the forum.
Everyone on the forum is essentially anonymous. You don't know for a fact who they are, what they look like, where they live or come from.
It doesn't matter when you're talking to someone about games or some random topic.
It shouldn't make a difference if you're talking to a 12 year old or a 14 year old or even a 20 year old.