I agree that 何歳だっけ would be fine. Using the "younger" version of the kanji, 何才だっけ, might be more "hip".
Man... cmt you're 19 and you want to feel younger? Some people are just too much
Do a search of the forums for the phrase "tattoo" and you'll see a ream of reasons why you shouldn't get a tattoo on your body in a language you don't understand. The long and short of it is:
a) It's not cool.
b) The artist will most likely get it wrong.
c) You don't know the language, so you don't know the meaning.
d) No, knowing the translation doesn't equal knowing the meaning.
e) For all you know, you could have "toilet" imprinted on you.
f) I've seen the above mentioned tatoo, as have a few other members here.
So... my advice? Don't do it. Just write it in English so it has more meaning for you. If you write it in Japanese, the only reason is because you want people to see it and ask about it. There are many more productive conversation starters out there, my friend.
