With drinking, we need to distinguish between two different types of people.
There are people who are actually kids, and then there are people my age.
I don't have a problem with people who are actually kids being prevented from buying. As you say, they don't know how much is too much, and they are too young to actually consent. Although I do have some issues with the enforcement of such a law (I'm not exactly thrilled with the idea of having to show ID and I think its a 4th amendment violation if the government is requiring it rather than the store.)
On the other hand, a 19 year old such as myself IS an adult. Now, I understand you can say a 25 year old has more life experience, and a 35 year old more than that, and a 55 year old more than that, and so forth. But at the end of the day, since I am an adult, its absurd that I can't buy alcohol (I don't actually want to, but that's not the point.)