Trump is almost 70, and while there’s nothing wrong with that, if you want to run for office, you have to stay current. He’s not, has no elected experience in the past to draw on, and is basically just a wealthy old man shooting his mouth off.
I'd discount the age factor in this case since Sanders, Biden and Hillary are in the same age bracket, Warren is 66. Republicans notably have some young guns but not all of them are youthful. With age comes some experience of having seen a lot of things. A good number of people probably don't have a good recollection of President Reagan.
Age means little to me in this because Reagan was such a solid President.
And both Kennedy and Obama were/have been lousy presidents.