Like I said in OP, whatever was closest in the drop down. I don't want to give them my real birth date.
With COPPA in effect in the US, you never want to pick an age under 13 or your account will be disabled.
To be safe, you also never want to pick an age so that you were under 13 when the account was originally created. Some services will lock you out for that too.
Oh yeah I forgot about that, I've heard stories of that happening (with PayPal IIRC?)