In my opinion, most people are using "old" as a proxy for "not fit for duty." It's just that no one wants to say the President is not capable of performing his job.
If an employee showed up to your workplace as incoherent as Biden was at this debate, they would be sent home and may need to undergo a "fitness for duty" test to return. While I agree selecting an even older candidate would be difficult, you have to admit Bernie would absolutely skewer Trump on a debate stage and no one would ever have to worry about his mental competence, at least until November.
Again, I'm open to alternative candidates - I've only seen one mention of Whitmer, but...eh... Who else is there?