Nightclubs, especially in Tokyo, have been cracking down on underage drinking. I was in Shibuya 2 weeks ago and we had a hell of a time getting into a club because my friend's gf forgot her passport.
Bring ID, particularly picture ID. Like has been mentioned, your passport is supposed to be on you at all time (I usually just carry a copy of the picture page and my current visa), and this can be requested from you at any time for any reason (or no reason) by the police.