Every home I've done for a wealthy person, regardless of age, had bidets. But they're also rich enough to hire people to take care of them as they age.
Coming from a working class background, the perception of "European-ness" is the exact - and possibly only - reason your average American old dude wouldn't use one.
I've talked with elderly blue collar working class Australian's who've encountered { bidet's | jet washing arses } for the first time .. once they've used them they almost all 100% don't need convincing that they're desirable.
It's literally an ad campaign | marketing problem .. and given how popular pressure washing youtube videos are, there's one angle to work from :-)
Probably Italy. From my experience in the Netherlands, a bidet is quite rare and requires a new toilet in some cases as exposed plumbing went out of fashion 20 years ago so a retrofit is not easy like with most North American toilets.
Up until a couple of years ago you were not legally allowed to build a house without at least one bathroom with a bidet, so essentially all houses built in the past 80 years in the country have bidets