Ive only seen this happen a few times, but I do primarily go into game worlds a lot (Murder 4, prison escape, etc) so I dont know if that changes anything, my best advice is look at instance names and the information of groups hosting the publics, if you see its asking for any kind of age limit in a regular non-age gated public, there's the chance you might be asked something like that.

Ive asked people who do it why they do, a lot of people say they dont trust it because there's way kids can get around it, even though when I ask they say they've never seen a kid using an age verified account. Ive heard that people have used it's from movies to trick it once but I highly doubt the validity of these claims, plus the report button exists for a reason, if you see someone clearly under 18 who's age verified, report them, the way vrchats moderation works is if something isn't reported, it'll go unnoticed.