Teen drinking will probably always remain a problem, but the extent of the problem might be minimized depending on how we handle the situation in a given society. For example, different cultures have different percentages of teen drinking problems based on how they structure their laws, how alcohol is typically introduced to young people, and so on. Some people argue that drinking problems are not as big of an issue in some European cultures where younger teens may be introduced to alcohol at a younger age and it is not as strictly controlled. Other people argue that it is better to have the drinking age kept up higher like it is in the United States in order to avoid problems.
Of course a big factor in all of this is driving. In many foreign countries, not as many teens are driving so early in their life, whereas in the United States most teens start driving at a very young age, and nearly every teenager has access to a vehicle. This makes for a much stronger case to have a higher drinking age up in the low twenties rather than bringing it down to 19 or so.
Average lifespan should be a factor in this equation. Maturity plays into this as well. Thousands of years ago, 21 years of age was considered middle-aged. People only lived to be 30 or maybe 40 years old. These days, life is just starting out at 21, and many people at this age are still quite immature from an emotional standpoint. Thus it would make sense to keep the drinking age a bit higher rather than lower it to 18 or 19, because we are living longer and maturing more slowly than we used to.
Trying to prevent young teens from drinking is especially challenging, since they are naturally going to be curious and they also are getting a strong message that “drinking is OK” because their elders are doing it. Therefore there is no good way to transcend this problem; younger people know that adults who are of drinking age are not inherently different than they are and have no special skill that allows them to drink more responsibly than a slightly younger person. Anyone who is one day shy of being 21 years old knows that they are not going to magically wake up the next morning and suddenly be more qualified to handle alcohol. Trying to convince someone otherwise is not only wrong, it also makes you look stupid.
The real “responsible drinking age” is determined more by a combination of biology and emotional maturity rather than simple age.