Yeah, I mean, the next thing you know you have old folks getting together and mugging people. Or gumming people.
There are essentially three reasons why society puts people in prison:
1. To protect society against those likely to commit more crimes if they are not jailed.
2. To rehabilitate prisoners so they can become functioning members of society.
3. To punish people for their transgressions. Don't do the crime if you can't do the time.
Of the three, I favor 1&2 but recognize that 3 is important in severe enough cases. Now think about Sheriff Joe. Likely to re-offend? No, he got voted out and at 85 I don't see him running again. Does he need to be rehabilitated? Not an issue. So it boils down to punishment. Remember, the "crime" that Arpaio was convicted of carried a 6-month sentence. In our society, that is not indicative of a severe crime.
So forget the comparison to 85-year-old Nazis. Looking at the thread, a lot of people want to sentence Arpaio over crimes for which he has never been tried. Guess what? Unless Trump issued a blanket pardon, you can still go after him for those things.