As a resident of a college campus and person with a general knowledge of how the world works, I know how little affect the drinking age has on people actually drinking. I once had a chemistry teacher who said, very aggressively, that your frontal lobe is not developed until you are 25, so drinking before then is a terrible idea. She then followed that statement with a long lesson on how to drink responsibly because she understood, as most do, that people will drink regardless of laws or science telling them they are too young. The current drinking age is seldom followed and even less often enforced, so educating underage people on safe drinking is a more logical approach than just telling them not to drink.
Conversely, as making things illegal tends to make them more desirable because of the air of danger surrounding them, lowering the drinking age might lower the number of underage drinkers because of the loss of those who drank just because of the thrill of doing something illegal. This might not happen though, as the drinking age is similar to speed limits in that they are technically the law and sometimes people get in trouble for not following it, but largely it is not followed simply because people do not view them as real enough to actually follow them.
The inevitable nature of underage drinking and the overall lack of regard for the drinking age make the actual age the limit is set at not matter, as whatever it is legally set at will not actually be cared about or enforced. That being said, a lower drinking age will lead to less 18-20 year olds having underage drinking as another charge against them and, as I previously stated, possibly reduce drinking within this age group. The drinking age being set at 21 does not actually make sense, as it is not 18, the age at which you become an adult who can join the military, get married, and buy tobacco products, or 25, the average age at which your brain is finished developing and when drinking does not have as harmful an effect on it. A better solution would be to educate people on responsible drinking instead of shouting them about not drinking until they reached an arbitrary age.