
it's the same as a nightclub
if you meet a girl in a nightclub that only allows 18+ year olds in, drinking beer that can only be served to 18+ year olds, smoking cigarettes that are only available to 18+ year olds, and the people who's professional career relies on them correctly verifying ages have agreed they are over 18 for all of this. You will still be arrested if you sleep with someone who's managed to slip through all of this by lying about their age.
It's one of those nightmare scenarios, you either ignore anyone who looks under 30, or take a chance on it.