I just saw this thread again and realized that my Mooney, now fix gear, was conceived in 1978 (with horizontal dropouts just so it could go fixed if I ever wanted to) and delivered in '79. 45 years old and qualifies for this thread. Been fix gear since 2017. Little danger it will ever go back.

My legs made the transition in 1976. Now the majority of my life's miles are fixed. (I did single speed 4 years before I went fix gear. Crashed first FG ride not thinking and coasting (with too high a seat). Still, got home and I was sold! Haven't ridden SS since.