Knowing the negative effects of alcohol, I don't see why you would want to lower the legal age, and I am talking about being able to BUY it. Even older adults themselves aren't fully responsible with the effects of alcohol in their system. As Thrill stated, what's done in other countries isn't the same as what's done here, but they also have different environments with different means of transportation. Exodus also mentioned the fact that many teenagers don't start fully driving until they are 17-18 years old, meaning that they are far from experienced drivers.
Saying lower the drinking age is basically like saying lower the legal age to buy tobacco products. The difference with that though, is that one physically and mentally impairs you. Both however are not healthy and shouldn't be consumed by kids at all.
The other thing with lowering the drinking age to 18 is that many of those teenagers are still in high school and live at home while college kids are out on their own. So just because we are considered young adults at 18 does NOT mean that most 18 year olds are fully developed and fully matured individuals. We are still TEENAGERS at 18 meaning we are still going make irresponsible decisions and judgement at this age.
News flash, 21 year olds are often just as irresponsible as 18 year olds, and make plenty of poor decisions with alcohol. However, there are a lot of 18/21 year olds who are very much the opposite of that.
I live with my boyfriend, who is 22, and have more "adult responsibilities" than a vast majority of early to mid twenties individuals that I know. It'd be great if my boyfriend and I could go out to a show in a bar or 21+ venue, or grab a drink together after work, instead of consistently having to spend nights in because of our sleep schedules and the law - whereas when we go to other countries, we can enjoy these things together. Fortunately forged documents exist, but it shouldn't have to be that way.
If you're worried about irresponsibility, realize that other countries with lower drinking ages. Are their teenagers somehow different than ours, and therefore it's not a worry? Transportation isn't the biggest topic in the drinking age debate, so it's kind of silly to focus purely on that. My friends who've grown up in countries with a lower drinking age express how much less appealing going out and partying beyond reason became over the years after being able to get a drink with their parents in a pub, and having alcohol and the risks be introduced and explained at a younger age.
It's hard to argue for a lower drinking age because it is so trivial, but I much prefer how other countries handle these things, honestly.