Mark Kelly is 60. The ideal age for a first-time presidential candidate is 57, so they can retire after two terms at 65. Mark is 3 years past ideal, but still 18 years younger than Trump is now.
He and his twin brother could both complete two full terms and still be 2 years younger than Trump is now.
Mark should be at the top of the ticket, though. He is far more competitive in the contested states.