I’m not the first to come up with the term, but judging from search results, maybe the third or fourth.

Toxic adulthood is, broadly speaking, insistently associating things with adulthood that have no good reason to be associated with it and judging or harming oneself or others on that basis. That includes not only the idea that adults should have power over minors, but also the idea that adults do or don’t do certain things. It’s the notion that adults shouldn’t watch My Little Pony, that adults should know how to drive a car, that adult toothpaste must be mint-flavored. It’s criticizing adults’ behavior as “childish” or saying that a “grown-ass man” should know better. All that sort of stuff.

This seems like such an obvious term, I’m surprised it’s not already in regular use. Similar uses of “toxic” are well-known now, so it might be useful for calling out such behavior in a readily understood way.