I agree, 19 is fair. At least most HSers are out by then.
Here in Ontario the drinking age IS 19. Bars, restaurants, the LCBO (govt-run liquor stores) and The Beer Store (govt run beer stores) are very aggressive about checking ID. In fact, a lot of restaurant/bars have regulations about turfing out anyone under 19 years old at 9:00 or 10:00 pm, when the bar-and-party atmosphere starts to gear up.
Growing up, I was always allowed to have a small drink of wine at special occasions, and at parties etc. under supervision, so it never had the appeal of an illicit activity for me.
I grew up during the time when they got much stricter about checking ID. When I was 14 I could waltz into any LCBO store and buy alcohol, no questions asked. I looked a lot older than 14, so no one ever asked me. By the time I was 19 and had proper ID, I got asked EVERYwhere.
Even at 14, though, I wouldn't buy alcohol for anyone else. I had my parents permission (well, license really. They weren't exactly paying attention), but others didn't, and somehow it seemed way worse to provide it for someone else than to get it for myself.
We don't seem to have a huge problem with underage drinking around here (lots of young people, 3 universities and several community colleges in my city, 5 or 6 high schools in my neighbourhood). The kids who live next door have a party every weekend when their parents go away. There is no raucous behaviour, not even blaring music. They hang out, talk and have fun. I suspect there is some drinking going on, but no rotten behaviour. We keep an ear out in case of trouble, but there isn't any.
I'm not sure where I stand on this, really. It strikes me that it really is about maturity, personal responsibility, parental guidance and expectations. Whether a person hits that point at 16, 18, 21 or 25 depends on the individual and you can't legislate it.
I just don't know...