Traditionally, when you were too old to work the farm or prove the theorem (sorry, cognitive decline is a well-documented thing, and tends to coincide with physical decline[1]), you would "retire" to a productive life in less-demanding pursuits: caretaking of the grandkids, woodworking, food preservation, writing, etc.
It's important to maintain hobbies in your youth such that when the time comes, you can turn those hobbies into productive and enjoyable enterprises. Certainly many would disagree, but such a path seems somehow "better" than popping Metformin in front of a television in a Boca Raton retirement community or tooling around the country in an 8 MPG RV.
[1] Yes, I'm aware that people like Dave Cutler are the exception to the rule. But they are, to be sure, the exception.