Ok, what do we have here ?
Main Entry: un·der·age
Pronunciation: "&n-d&r-'Aj
Function: adjective
1 : of less than mature or legal age
2 : done by or involving underage persons <underage drinking>
Of "less than mature age" = you cant pinpoint a precise age for the term "mature"
and "less then legal age" = any age from 1 to 21