Interesting tidbit on alcohol affecting specifically people of young ages: There's actually been no studies done to confirm this.
Outside of the extremely rapid cell formation while a fetus, alcohol doesn't have any different of an effect on youth than it would on adults (that is, a fairly negative effect).
Alcohol drinking ages are in place in Western Nations because the bible placed restrictions on alcohol. There isn't anything morally wrong with a child drinking when compared to an adult.
I guess you could make the argument that adults have more reason and are therefore less likely to become addicted, but I would challenge that by pointing to any university. Ever.
EDIT: I actually just read a study which concluded that there were LESS long-term negative effects on children because their internal organs (namely their liver) undergo healing processes much more rapidly.