I think that to be eligible to be president, your age as of the last day of your next term should be less than the life expectancy of your constituents. You want to serve longer, you better make sure your constituents live longer.
As far as Biden stepping aside, I don't think it's a bad idea at all. Find a 40- to 50-year-old combat veteran who grew up with wind turbines and solar panels on her family farm, and has been living and working in a blue state for the past 5+ years. Have her run on a platform of "Ok, Boomer".
Juxtapose a picture of her running with a pack and rifle against a doctor looking at X-rays of a foot with a magnifying glass. Photos of her family's wind farm against headlines of Trump denouncing wind turbines. Her smiling at an award or promotion ceremony against Trump's scowling mugshot.