It's been my experience, at least here in Chile, that younger guys are into many things though you have to get them talking about it.
Same in my neck of the woods. Most guys in my age range (24, 25) usually have a fetish or two they want to try in the sac, but keep quiet about it if they don't aligned with more..."traditional" bed activities. I think a lot of the guys I know consider it too extreme, mostly for what Arx said:
i find younger guys hide their interest in it for fear of appearing too "loose"/depraved.
Though I think they fear being "too loose" in the literal sense. I've talked about it with some guys before and get the sense they think once you do something like fisting there's no going back for your hole.