the older i get the more i like mum tums

I think people like watching things they can relate to / fantasize about doing.

So i would think old people would enjoy watching old people, at least some of the time. Kinda like 30 something moms who are really into dad bods. Or all the milf porn some middle aged dudes like.

Nah. People’s preferences are varied and for lots of different reasons. That part made me smile, though.

I'm going to make a few assumptions here, but my thought is that women in porn largely look like they're between the ages of 18 and 30. Men on the other hand look like they're 30-45 even when they're 'acting' like they're a lot younger.
Guys in porn are usually quiet and the focus is on the woman because we're supposed to be able to imagine ourselves as the guy. Kind of like the master chief. You aren't supposed to see his face because you are him.

These Japanese guys want bodies that resemble theirs more, so they can think of themselves in those positions easier.