Underage drinking may be less serious than DWI/DUI, but it's a completely different charge, rather than a lesser included charge or a something which is de facto (even if not officially) a lesser version of DWI. Whether you are an underage drinker has nothing to do with you are DWI, and vice versa. So, for example, say somone broke into a house to rob it and in the process killed someone--he'd be charged with both manslaughter/murder and with breaking and entering, since they are wholly separate offenses.