[BFritz]

Teenagers are too immature to drink alcohol responsibly (they do now, but making it legal to buy would make it so easy and so much more common).

Too many accidents like what happened to me would happen (friend hit a tree, gave me a traumatic brain injury, I was pronounced brain dead, etc etc).

Nobody mentioned the medical part of it- your brain's not fully developed at 18 (or at 21, really, but it's closer), and alcohol kills brain cells, so you'd be killing them before you would even grow enough to have a resonably properly functioning brain.

Kids at 18 would drink all the time just to get drunk, as opposed to a lot of people of legal age who just casually do it to relax and hang out.