The problem isn’t so much with people’s supposed maturity and more so with the fact that an adult can get in BIG trouble for interacting with a minor in an “inappropriate” manner even if they were unaware that said individual was a minor.

Exactly. 18 is of course a somewhat arbitrary legal fiction that sweeps up individual cases. Any such line when dealing with people vaguely in that age range - 16, 18, 21, whatever - is by its nature going to be so. Nevertheless, 18 is what we have, and the legal risk involved is simply too high.

They brought up supposed maturity because that’s a common justification used by minors to be in adult spaces.

This^^ I say this so much