I'm not sure if there are solo founders over 50 who were at YC but some individuals who are cofounders are. Dr John Slough of Helion for example.

I've got similar issues myself. Perhaps cofounders are the answer.

Maybe we can take inspiration from Rose Blumkin, founder of the Nebraska Furniture Mart who sold the store to Warren Buffett and then after arguments with her family left in a huff and set up a rival called Mrs. B's Clearance and Factory Outlet across the street at age 95. It became profitable and was Omaha's third-largest carpet outlet before Buffett bought that too.