The voting age was lowered because of the Vietnam war. A lot of people thought that if they were old enough to be drafted and sent off to fight then they should also have the right to vote. So we got the 26th amendment.
“old enough to fight, old enough to vote”
https://en.wikipedia.org/wiki/Twenty-sixth_Amendment_to_the_United_States_Constitution
Yes, I was there. That was the excuse.