The young being leftists is new, it started in 1992.
Nixon won 52% of the 18 to 29 year old vote in 1972.
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.
In 1980 Democrats got 44% of that age group, 1984-40%, 1988-47%.