Overtime use of alcohol at a young age will make you become tolerant which will make you drink more thus damaging your insides and possibly making you become addicted.
1) It can damage his/her health and cause death at a younger age than normal.
2) It can affect his/her progress in school
3) The teen can get in trouble with the police, if he/her is underage, it can bring costs and even juvenile prison if the teen causes trouble while drunk