When do age gaps become a problem?

That's a pretty interesting question and the reasons people give for why it's not always ok even if everyone's a consenting adults. We're supposed to be living in an era where you do you as long as it's with a consenting adult. But suddenly there are these people who talk a great game about being open-minded, but come up with these exceptions. When they try to rationalize them, all I can hear is "it's not ok if I don't like it"
Oh and one such exception I heard was over extreme age gaps among consenting adults