Drinking alcohol is bad for kids for LOTS of reasons. By the time you are a grown-up, your brain has pretty much finished developing. But kids' brains grow everyday! Exposing the still developing brain to alcohol can cause lasting harm. Drinking can affect your ability to think, learn, and get good grades in school.
Drinking alcohol can make you feel VERY sick. Your head may hurt and you might feel so dizzy that it's hard to stand up. Your body may try to get rid of the alcohol in it by throwing up. Alcohol can even cause death.
Kids say their parents' disapproval of underage drinking is the main reason they don't drink. That is why your parents probably have rules for you like: NO drinking alcohol and NO giving, asking, or taking alcohol from brothers or sisters or anyone else.