Why 21?

Why is the drinking age in the U.S. 21? It used to be 18 and in the majority of countries around the world there is no drinking age. Don’t people realize that by making alcohol not illegal to kids under 21 it loses some of its luster. Sure a lot of kids would still drink it but it would not be as fun to drink. People make the argument that if your not 21 your not mature enough to drink. Does that mean kids in other countries are more mature than kids in the U.S. and more capable of making smart decisions???