Here's my combined list:
GOP:
Donald Trump 74
John Kasich 68
Mike Pence 61
Ted Cruz 49
Dems:
Bernie Sanders: 79
Joe Biden: 78
Elizabeth Warren: 71
Sherrod Brown: 68
Al Franken: 69
Cory Booker: 47
Gillibrand: 49
Duckworth: 48
Harris: 52
Klobuchar: 56
You're mixing up their current ages with their ages in 2020. E.g., Sanders will be 79 in 2020, but Booker will be 51, as he's 47 now. Gillibrand will be 53, as she's 49 now. Kamala Harris will be 56 as she's 52 now, etc.
Also, I maintain that Justin Amash is a lot more likely to primary Trump than either Cruz or Kasich. Amash will only be 40 in 2020.
The younger Dem's were just copied from Lief's post. Guess he was trying to prove his point by hook or by crook. And I agree with Averroës that these figures are outclassed in political skill and voter appeal by their elderly peers. (Seriously, Murphy?! Cuomo?!)
Amash hasn't been talked about much around here recently so I didn't think about him.