Stanford changes alcohol policy

"http://responsibility.org/get-the-facts/research/statistics/underage-drinking-statistics/
8th grade students who reported drinking in the past month declined 61% from 1991 to 2015
10th…declined 50%…
12th…declined 31%:

If the drinking age has no effect on limiting kids’ drinking, how do you explain this? If the drinking age has no effect on limiting kids’ drinking, why bother to have one at all?