I do have a hard time with the idea that those who are active duty military and engaged in wars around the world cannot legally buy a Bud Light. I also think perhaps an exception could be made in the law, on military bases, that includes active duty 18 - 21 year olds.
It's also legally problematic to exclude those 18 and over from legal alcohol purchases, while still being legally considered adults in almost every other way. 21 also seems like somewhat of an arbitrary age.
Since we have such a large amount of people who abuse alcohol in this country, at this point if we were going to lower the age it would just make it more dangerous for younger people to get involved with drinking.
One common reason people state for the drinking age to be lowered to eighteen is that citizens can vote and join the military at age eighteen. Therefore, if Americans can participate in their government by voting and fighting in wars, why can't they drink at the same point in their lives? If someone can legally be killed defending his/her country at eighteen, why can't the same person have a beer? Some arguments suggest that anyone serving his/her country "deserves" to have a drink. There is also the idea that if drinking isn't such a huge deal, people will drink less - in other words, if drinking isn't forbidden, it won't be as exciting.
Drinking and particularly excessive drinking is more harmful to health of children then of adults. Also children by nature are likely to be less cautious in the amount of drinks they consumed, and their behavior after drinking. For example, it is more likely that in a party they are more likely to be less cautious about limiting the amount of alcohol they consume, hand having consumed excess alcohol they are less likely to avoid driving. In view to these consideration it is in the interest of children to put some age limit before they are permitted to drink. The exact age limit can be varied to some extent. I personally believe the minimum appropriate limit for this is 18 years and maximum limit is 21 years. To some extent the appropriate limit will also depend on the culture and general health standard in the country imposing these age limits.