There should probably still be drinking ages in public places (municipalities or states deciding these, of course), but a parent should not get in trouble for serving a beer to his or her minor kids in the household (however, they would be, like always, responsible for any actions their kids do if they leave the household afterwards). When I was a teen, I drank with my Dad--moderately, like a beer after we'd been working together in the yard or something. It was a good bonding experience. Technically this is illegal, and these days I've heard of incidents where the law in such cases was enforced. Which I think is silly.
I also agree that if one is in the military, they should be allowed to drink (off-duty of course!). Throughout history, in the traditions of armies and warfare, drinking served as a sort of vehicle of comradeship and esprit de corps, a bonding ritual that at the same time released some of the tensions of a perilous and demanding job. Drinking helped keep morale for most soldiers anyway. Of course, any unacceptable behavior that may result from drinking should be punished just as unacceptable behavior from a sober person is. This teaches that even under the effects of alcohol, the soldier must maintain the basic discipline of self-control--he is responsible for his actions. And this is true for anyone.
I said those last few sentences partly because of a development in military policy in my last couple years in the Navy. Before I came to Japan, an incident had occured in Okinawa where a group of four marines and sailors raped a 12-year-old schoolgirl. They had been drinking, so subsequently DRINKING and ALCOHOL were blamed! This was an excuse to crack down on drink in the military, particularly overseas. It eventually became an arrestable offense to be WALKING FUNNY after an evening at the base club or out on the town--"public drunkenness" was defined down to that minute deviation from sobriety, never mind if you weren't causing trouble. And 20 (the Japanese age) became the rule on base, with no grandfathering.
My argument with this whole approach was that, in a way, it served to remove the soldier's/sailor's responsibility for their actions, by blaming a substance that can't think or decide. In a way that they probably never intended, it served to relieve some responsibility from the perverted sailors who raped the 12-year-old (yes they were still prosecuted though--although I think the Japanese authorities should have been given the honor in that exceptional case). A person who rapes a 12-year-old little girl has MUCH MORE wrong with him than a drinking problem. Surely that is obvious....