Tell you what... I was pretty hammered from about the middle of my Sophomore year until I turned 21. That's about when it lost its appeal. I'm talking full on 'CBS special on binge drinking' black-out drunk. There were days I'd come home, at 17, and pass out on the kitchen floor with the front door wide open. In Buffalo. In February.
The alcohol wasn't very difficult to get at all. There was always an older brother or an older sister. Hell, stand out front of Wegmans asking guys at random and you're bound to get someone to do it for 10 bucks. Here's a hint: ask the scummy looking bastards.
It got even easier when a 21 year old friend gave me his ID. Ridiculously easy. I remember walking into gas stations at 2:00 AM barely able to stand up. The clerks don't care. I had an ID so I'd walk out with a couple of cases. It never failed. I must have done that 50 times.
My opinion? Keep it 21. When I was a sophomore in HS, half of the kids I was partying with were 18. We called them seniors. Hell, if we're going to change anything, increase the age for the *other* stuff. Or make everything 20. Throw driving in there too for all I care. Military, voting, drinking, tobacco, binding contracts, make it all 20.
For the most part, there's a separation between the legal crowd and the underage group. It happens naturally as the older crowd heads off to college and whatnot. That does wonders to keep teenage kids somewhat sober.