FWIW, probably just a coincidence, but every 16 years (D) party has a younger fresh face outfront: Kennedy ('60), Carter ('76), Clinton ('92), Obama ('08), Harris ('24). Only works in most recent postwar era though. Why? Generational? But 16 years seems bit short to define generation...

...except conventional demarcations between boomers gen x, z, etc tend to average somewhere in vicinity of 16 year intervals between them

And BTW, the '76 cycle had another fresh face contender named Harris...