It must not be forgotten that "underage" actually isn't the same everywhere. To make an example: US drinking age is 21. Norwegian drinking age is 18 (20 for strong stuff). In Norway the rules concerning sex are based around the age of 16. (yes, that means a fifteen-year old can't have sex with his/her sixteen-year old partner until he/she is also sixteen.

A better question, we think, than the "are you 18+?" is "do others consider you mature?"