I'm just got back from Japan (I am 19) my first international trip was when I was 17 to Iceland, so don't worry your not alone at all. It's a good thing believe me.
First of speaking the language will be invaluable!! I wouldn't to worry to much about your age, as a foreigner who speaks a decent amount of Japaneese(I assume) you should be in a fairly decent place. Cabs are really expensive in Tokyo so learn to use the subway system, it's great. Also convience stores are good for cheap eats, not bad at all, also check out the restraunt on top of the department stores in Ginza, I don't know if you need somewhere to stay but I know a few good hotels which cater to Japaneese buisness men which are much cheaper then those that cater to westners, thier even cheaper then most hostels, and you get a decent room. PM if you want the contact info.