Here's an opinion post actually relevant, so I consider diaper fetish to be mostly a pants fetish, at least for me personally. I've also heard it called an underwear fetish but I consider diapers pants so for me it's pants. I've seen a lot of people treat diapers as a baby thing, which is pretty obviously okay, except some people are too set in the mindset of seeing them as a baby thing and refuse to consider that there's lots of other ways to like diapers that have nothing to do with baby stuff. Those people really take offense at people saying that it's an underwear fetish, which I think is ridiculous because staking your emotions on whether strangers online conform to your exact perception of a fetish is really foolish.

Diapers are just pants designed for pooping, and pooping your pants is an all-ages activity. No need for it to be associated with younguns unless you want it to be! For the record I like ageplay stuff, but my main fetish will always be shitting. Adults casually dumping their diapers is my favorite thing and I think it's beautiful.