It wasn’t until the mid 80s until many states raised the drinking age from 18 or 19 to 21. In 1984, Congress passed the National Minimum Drinking Age Act, which required states to raise their ages for purchase and public possession to 21 by October 1986 or lose 10% of their federal highway funds. Wyoming was the last hold out and raised the age to 21 in 1988.

One reason why teenagers overall drank less in that time period, is it was less likely to have a 21 year old friend when you were in High School then a 18 or 19 year old friend to by you alcohol. With time the culture changed and high school kids found other things to do with there free time, including smoking Marijuana.