You’re still using the 18-29 figures. It’s possible that the year after the <21 were allowed to vote they actually came out, but historically it’s been the worst age group on turnout. That decline after the initial burst may be attributable to the end of the Vietnam conflict and draft. Not having any skin in the game does that to people.
So having "skin in the game" made the draft age people vote pro-war, and pro-Republican? Did you take note that the under 30 age group was always the most pro-Vietnam?
You keep talking about 18 to 21, why is that, you don't have any numbers for them, that is an incredibly thin slice to find numbers for, and you were totally wrong about the youth vote turn out in 1972, it was the highest in history, contrary to you thinking they did not show up.
The 18 to 24 year old vote (which we do have data for)in 1972 was over 50% also, again, the highest for that age group in our history.