It is said (among those that I know) that allowing a minor to consume some alcohol actually encourages responsible behavior when older. The explanations vary, but most have concluded that the removal of the stigma removes the mysticism from the product and therefore decreases the desirability. This is followed by the argument that this will encourage the child to learn to drink responsibly in the home as opposed to in a less controlled environment.
I am aware of a relationship to this question, but that has to do with legality over illegality. I am more interested in stigma.