I don’t know, when I was barely 20 I went out with a boy (? he’s not really a ‘boy’ anymore) who was 28 going on 29. I have been described as mature by most people who meet me, which is why the shidduch was redt. I was really uncomfortable with it from the outset, but people told me not to limit myself. So I went out with him once, and I was still uncomfortable, so I said no (he said yes).
My parents have an age gap of 10 years, but at this point I can’t see myself with someone more than 6 years older than me… obviously, it depends on the person, so I won’t put a hard cap, but I’m very hesitant to go out with someone who’s a lot older.