Technically, it's 20, but just like in the U.S., underage drinking is common, especially in college. Drinking is part of Japanese culture, and there isn't such a puritanical attitude towards it (which also means that while underage drinking isn't exactly approved of, it's also not considered such a moral hazard), so I don't think Japanese kids are as likely to go out and celebrate being able to drink "legally."
Oh and what is the legal drinking age in Japan? I think its 20 but I might be wrong. Also, when Japanese teenagers become old enough to drink, will they often go out and drink (a lot) to celebrate like a lot of American teens do? Thanks.