If Strom Thurmond can father kids at 68 and up, Doc Brown should be able to as well. 
Huh. I’d never noticed that before.
At the start of BTTF2, he tells Marty that he was rejuvenated, had his blood replaced or something, which added thirty years to his life.
PSXer
24
Also, Doc had that rejuvenation thing done in the 21st century, so he is not too old for Clara
edit: yes I see someone above me already posted that