It depends, really. In the US it's boys and girls before 21, which I'm inclined to agree with. I call all my 19 year old students boys and girls. I do take your point that grown adult women are often called "girls" for no reason other than misogyny.
My point has nothing to do with underestimating a 19 year old female's agency just because she's female though. I would equally safeguard a 19 year old boy with a 30/40 year old woman. In both the 19 y/o female/male's case, ultimately I'd have to respect their decision, but I'm sure many posters would have reservations too.
The reason I mentioned females: the posters' logic is that "girls are more mature than boys (as they develop), so therefore many 19 year old females are more mature than 30+ year old males". Yes, they said "many".
By their logic also, they would safeguard their 19 year old DS (since males apparently lag behind in maturity by over a decade) but not their 19 year old DD! What on earth?!
That's biologically incorrect logic, and awful. You're putting a big burden on females to be much more mature than they are. It's the "hehe boys will be boys mentality" only inflicted in reverse upon girls.
I don't actually believe those posters would be happy with their 19 year old DDs marrying 30+ year old men though, which makes this thread a massive waste of my time!