Actually that's incorrect. There are technically two drinking ages. One at 16 for beer and one at 18 for all other alcohol. There's even a couple of counties that have an age of 14 for beer. Some are just 18.
As for my opinion. I personally think having a drinking age of 21 is silly because the vast majority of kids would have had alcohol or even got drunk long before that. As someone pointed out you're considered an adult at 18 legally so what's the issue?
Saying all that, from a health point of view, something most young people don't think about, it's very damaging to you mentally and physically. Most mind development continues until 25ish but keep in mind that your mind never stops changing. On top of that alcohol damages liver function, blood, etc if used excessively, which most young people do. The idea of a party without excessive drinking to most young people is just silly. That's part of the problem.
I've never been a big drinker myself and I only drink with company or on occasions. My wife and I have the rare occasion where we'll have a drink together of wine or something else.