Generally these people already have a sense of adulthood, and they believe (and probably are) old enough to make their own choices. They most likely choose to drink underage and are responsible enough to understand the consequences, and what can happen to them.

However some of these choices can affect the lifestyles of other people. For one those who underage drink and lose control can disturb other who are studying or sleeping during the night. They can be belligerent and can possibly destroy property and hurt other people. Also there have been lots of problems with underage drinking and driving. Many states have risen their legal drinking age because of underage drinking and driving related fatality rates.