Intentionally excluding Third Parties here is kinda weird given they actually on average nominate far far younger candidates. Other than the disputed Willy Hope Harvey who was originally on a third party before getting ousted and going independant, all the other old farts are mainline. Trump, Biden, Reagan, Dole, McCain, Robert M. La Follette(Bull Moose Guy Post-Teddy) and…William Henry Harrison? Ok didn’t know that last one, clearly not the right move though.

Also both John Adams and his son were shockingly old the last time they ran by the standards of the era, John Adams died at 90 and was the oldest living ex-president from the day his term ended(George was already dead midway through his first term and John Adams was older than him by the time he was midway through his second term) until 2001 when Dementia Riddled Reagan limped ahead and took the record before dying at Age 90, then Ford beat his record for 5 weeks in 2006 before croaking, then HW beat his record in 2017 and held it for a whole 6 months before croaking, and Carter’s had it since 2019.

Though accounting for life expectancy of the era John Adams is still insane. Adams had untreated thyroid issues which almost killed him a couple times, he had Typhoid in his 80s, and he got a kidney infection in his final months which helped finish him off. All of those would be treatable today so he’d probably be closer to Carters level by our standards.