I'm twelve. I'm also pretty sure I'm as straight as a limp noodle, or at least a pencil when I wave it about between two fingers, but the thing is that 12 is not an ideal age to come out. When should I? I told a friend once but it's more of a joke between us, she didn't take it seriously.
There's no rush. 12 is quite young but if you feel completely sure and you're ready then go ahead. Just make sure you know it's what you want because there's no harm in waiting a few years and once your out its hard to take that back.