Limits based on age ignore obvious differences in human behavior. People should be allowed to prove their responsibility based on either objective test criteria or approval by local community representatives. Other countries have much more lax laws and enforcement about drinking ages without severe consequence in their populations.
It would be good to see legislation that requires citizens to take a test that shows they are properly educated about how to drink responsibly. They should be made aware of the health risks and benefits associated with alcohol. There should be a system for removing qualification based on irresponsible actions. It makes sense that alcoholics should have their ability to purchase alcohol removed and responsible teenagers should be allowed to imbibe.
By granting increased privilege to responsible teens, we create an atmosphere where social lubrication is a moderated and public activity rather than a secret and unregulated activity.
A drinking age of 21 could be considered a form of discrimination, when you can smoke cigarettes and be sent off to war at 18. We're told that we are adults when we hit the age of 18, but still restrict certain behaviors to 21. I'm sure casinos would love the drinking age lowered, so they can offer gambling to a young set of ducks. Like almost all forms of prohibition, those that are underage and want alcohol will find a way to get it.