It's not considered safe for a kids or persons health to drink alcohol before age 18 because it can affect the way the brain develops and also could lead to alcohol dependence and the kid could decide drinking alcohol is a good thing and decide to get drunk.
I would never allow any kids of mine to drink any alcohol before 18 or even before 21 years of age.
However once they reach 18 then you can't legally stop them from doing something but the law can stop them from drinking before 21 as they can get cited and get into trouble if they are consuming alcohol or in possession of alcohol before 21 years of age.
The consumption of alcohol at a young age can impact how a teen's brain develops, disrupt their sleeping patterns, delay puberty, make it harder to concentrate at school, and even increase their risk for liver and heart disease, high blood pressure, and certain types of cancer.
The drinking of alcohol as a young person can affect memory function, reactions, learning ability, attention span, mental health, your chances of having serious accidents, disturbed sleep and liver damage – all of which will almost certainly impinge, to some extent, on your ability to function at.
So those under 18 should avoid alcohol as it's not really healthy for you.