Underage means younger than the age the law allows for something — drinking, buying cigarettes, driving, voting, marrying, going into a bar. The exact age is never inside the word; it depends on the country and the activity, so a person can be underage for alcohol and old enough to drive on the same day. It works two ways. It describes the person — underage drinkers, she is underage — and it also describes the act done by such a person: underage drinking, underage gambling, underage marriage. That second use is very common in news writing. Sport borrows it too, in a different, neutral sense: underage teams are age-limited youth teams, and being underage there is simply how you qualify. Usually written as one word; underaged appears but is much less common and looks wrong to many readers.