First election year that I was legally eligible to vote in after turning 21, was 1968...Nixon v. Humphrey.
In 1968 I was four years old.
Some trivia.
In 1968 the Democrats won 47% of the (21) to 29 year old vote, in 1972, they won 46% of the 18 to 29 age group, in 1976, after Watergate, and with the weak, moderate (and under suspicion) Ford, against what appeared to be a social conservative Democrat with a strong military back ground, the Democrats picked up 51% of the 18 to 29 year old vote.