I’d be super happy with no upper limit on age.
What I definitely have is an attitude limit; I loathe it when sullen teenagers knock the door, mutter “trckotrt”, no dress up except someone has drawn a tear on their face and then grabs five portions of candy and just dashes out.
Like, you can be fucking 40 for all I care, but you squeal “triiick of treaaaat”, then I say “wow, aren’t your costumes great” and offer the bowl up. You then grab one large or a couple of small things, say thank you and walk off excitedly.
The requirement for me is that you look like you’re enjoying it. Otherwise, why am I opening the door to strangers and offering them sweets?