You might want to create a new e-mail address (preferably from a different domain, e.g. if your current address is
xxxx@gmail.com, make a new one at yyy@yahoo.com), and e-mail them asking if they would let someone who played at their site while under 18 withdraw their funds when they turned 18.
If you play at a smallish site, then this might be kinda risky. I don't think they often have a lot of people's funds locked up, so it might arouse a little suspicion.
Getting a fake I.D. or altering the one you already have is a good idea too.