My grandma could have mustered a single jig and one day of walking in her 90s. She definitely would not have been able to work a part time job. I think you just want an excuse to hate grandpa Joe, so you invent one even though on proper scrutiny it doesn't hold up to reality. But you don't need to justify hating someone, you're allowed to just hate them for no good reason. Instead of throwing all old people under the bus for the sake of hating Joe, just let the hate flow through you without needing to find a reason for it.