I don't think your last paragraph is true. I remember as a lad (pre-Vietnam) that New York had a drinking age of 18, but that the surrounding states were 21. So there would be a lot of driving across state lines, and as you can imagine, a fair number of accidents, some very tragic in loss of life to kids who had their lives in front of them. The surrounding states never did lower their drinking ages. New York raised its to 21 after the Supreme Court upheld an act by Congress that said that states with a drinking age of less than 21 would be barred from receiving most Federal highway funds.
I think you are thinking of voting age. That did go down from 21 to 18 during the latter part of the Vietnam war. It has proven less important than it was thought to be because young people generally do not turn out to vote. (BTW, giving women the vote was also expected to radically change politics, and it didn't, even though female turnout rate is at least as high as male).