hehe. The above answer sounds interesting. " How old are you?" is absolutely correct grammatically and most frequently used to ask age. But for me or for most women, it may be a little bit implolite question, especially for the ones whom you meet for the first time. So I recommend not asking it to them. :)

Actually there's a lot of ways to ask that. You just need to be a little more creative. "For how long have you been living on this planet?" "How long time has past after the day you was born?" etc. :-))