The honest answer is simple. If your entire life can dramatically change in one single day, something is wrong.
Depending on your state (Obviously US), your going to be getting behind the wheel at 16, maybe 17. Your going to learn how to vote, pay taxes, and do other fun things at 18.
At age 18, you're just coming out of High School. You're just getting your first decent paying job, you're getting your girlfriend that's actually going to stay with you for awhile. You're now driving to places you never been to as a child.
Your experience with alcohol consumption is maybe being a Designated Driver, and driving to a bar, picking someone up, and driving them home.
You want alcohol consumption to be at 18 again? What the hell is wrong with you, as frank a statement as it can get. Do you have any idea how many people that's going to kill? Do you have any idea how much insurance is going to go up? Do you have any idea how many lives that will screw over?
You can argue that the drinking age was 18 way back when. You can't argue that in today's world we are much different then our older counterparts. We have Cell Phones, cars that go over 130 miles per hour straight from the factory, parents that are willing to just bend head over heals to buy kids whatever they may need, including a Mustang for their first car.
We have enough teen deaths, between inexperiance, cell phones, texting while driving, innatentive driving, speeding, road rage, driving in bad weather(Snow,Ice,Hail,Winds,Rain) and drug use. We don't need to kill more people just so you can drink yourself into a stupor.
Now, with that being said. Alcohol isn't that bad. It's a social drink. You go to a pub, and you have a few drinks with your good friends while watching your favorite sports team lose again. And then you drive home after spending 40 bucks on beer, which honestly, isn't that much for one person.
In the U.S, 60 percent of all teen deaths in car accidents are alcohol-related. That is not a bullshit fact. That is a true statistic.
Now, that's with Alcohol banned until your 21. I sure hope you are smart enough to know that that number is going to skyrocket if it's under 21.
And yes, I know, you are careful. You are not like them. You drink at home, your with friends, you feel like you are missing out on a social event every weekend. I know that you are not going to die in a car accident. I know that you can hold your own and still think clearly after a couple beers. But everyone is not like you. They ruin it for you by simply not being as good enough as you*.
Anyways, all in all. Yes, it would be cool for us under 21's to get some beer and laugh so hard we fall off of a stool. No, I'm not getting my sober ass killed by your drunken friends at 3am when your doing a hundred down the road.
*I've used 'you' as in 'the reader', not 'the first post'*
(All of this is based from a United States angle. Seeing as every single 'western' country is getting more and more like the US every day. It's angle applies more and more every day.)