Of course when the job is physical it's different, but isn't the problem that people don't like their jobs anyway, irrespective of their age?
I love my job - most of the time.
However, as I've got older and have developed some health issues and dealing with perimenopause on top, some days it's hard, and getting harder.
I'm not sure I'll be able to keep on working FT for another 10 years, let alone 20!