sadly the full "rationale" for this is that till your body exits puberty usually by a persons 21st birthday they are still considered "growing" and as such alcohol may interfere with the process. So to "save" the future generations our fore-kin decided to prevent future generations from "doing self-damage" to limit access. it took a special panel in the early 1900's to allow folks from other cultures to allow minor's Extremely Limited access to alcohol and under VERY limited circumstances. Several families before the special dis-compensation had parents arrested for child-hurt from overly protective neighbors butting in without a lick of cultural sensitivity.
Yes, except... repression doesn't work. NEVER works.
By cutting legal access, you create a black market. And worst, you create a fetish.
Just see the results of the "War on Drugs", compare U.S. results with (for example) Portugal with de-criminalization of consumption (along with several social measures).
The solution is to never forbid access, just help minimize the excesses. By creating a fetish, they drove teens to alcohol, the same way that where sex-education (the real one) is forbidden teen pregnancies are WAY more common. Not only because they don't have knowledge and access to good sex practices, they also live under a restrictive social code that fetishises sex by condemning it.
I could explain further, but this isn't the place or the time.
Peace
