Already tried it where? The drinking ages have gone up but I never heard of them going back down again.
The drinking age of 21 has been a complete disaster for all the reasons cited above. Under 21 drinking has been driven underground, forcing young adults to drink away from responsible adult supervision, where they often get into more trouble. The "forbidden fruit" syndrome encourages young adults to drink more than they normally would had they legally had access to it, because they do not know when they might have access to it again and thus want to take full advantage of the current situation.
This age 21 madness took hold in the early 1980s when I was serving in the Marine Corps. The military decided to keep the beer flowing in the enlisted clubs regardless of age, because otherwise, servicemen might go out in town and get into trouble. It was a smart move and many lives (not to mention careers) were undoubtably saved. When young people are drinking among elders, they have pressure to drink more responsibly because they don't want to be perceived as lushes or losers by people who they respect. Get a bunch of 18-19-20-year-olds together with a keg of beer and no elders around - which is what happens when you make it illegal for elders to be around - and you have the ingredients for disaster.
People do not magically become responsible drinkers at the arbitrary age of 21. It takes most people a few years to learn their limits with respect to alcohol. It is so important to have responsible adults around during these years. You don't simply hand your kid the keys to the car when he turns old enough to drive. You spend a lot of time with him first, ensuring that he or she is comfortable behind the wheel. So it should be with drinking as well.
Then you're not very old. The drinking age in most states was 21 years old prior to the 1970s. During the Vietnam War, the argument was often heard that if we expect kids to fight for their country, we ought to allow them the privileges of adulthood. During this time the voting age was lowered to 18, and many--probably most--states began lowering their drinking ages.
I went to college in Wisconsin, a year or so after the age was lowered to 18. I distinctly recall a Friday evening ritual at the college rathskeller called "Senior Drink." It was so-called because previously of course you had to be at least 21 to participate--the typical age of the senior college student.
The name stuck even though now, all could participate.
By the end of the 1970s, the statistics on traffic fatalities involving drunk drivers ages 18 to 21 were up so alarmingly that the states took another look at the trend.
Congress acted, and forced the age back up.
You need to remember (before you start yelling about Democrats) that this was the time when the Democrat party was considered the "party of youth," and the Democrats were the ones who had spearheaded lowering both the voting and the drinking ages.
So this was a popular mandate. People got tired of the bloodbath, as I mentioned.
So now you know the history that occurred BEFORE (apparently) you were born--since I've noticed a lot of FReepers seem to think that nothing much of interest happened before their date of birth.