OK, ceteris paribus, lowering the drinking age will make alcohol more widely available to kids. Yes, they CAN get it now, but there are SOME barriers, which means that overall obtainability of alcohol by people 18-20 will increase.

Now, would the HARM done by this age group increase as a result? Having to obtain alcohol illegaly changes the incentive structure of the situation. There is a greater incentive to binge, and people are much more likely to get drunk when they drink at all. Lowering the drinking age would make 18-20 years olds drink less heavily when they do drink, and basically behave more like the rest of us with respect to alcohol.

I am not sure which way the overall cost-benefit would fall out. It is certainly not dead obvious one way or the other.