We're talking about totally different circumstances here. Brazilian law doesn't see people above 14 and below 18 as adults, but it still technically allows them to be sexually involved with adults unless you can prove sexual assault. That's obviously wrong in any context.
On the other hand my problem with the take people have in this thread is that a "not fully developed adult" = "a literal teenager/child". These aren't equivalent, because I do think people over 18 are adults.
It's not though? As others have mentioned, cognitive development isn't done at 18.
It's not about viewing anyone as 18-24 as a 'literal child/teenager' (though 18/19 years olds are literally teenagers. It's right there in the number), it's about viewing them as incomplete adults.
Children/teenagers/young adults are not viewed as some monolith. I don't view a 10 year old the same as a 16 year old. I don't view a 16 year old the same as a 18 year old. I don't view an 18 year old the same as a 21 year old. I don't view a 21 year old the same as a 25 year old. We know as you get older your brain continues to develop, and adult responsibilities/rights are granted at different stages (right to drive/vote/drink/etc.)
If it's scientifically proven that brain development doesn't stop at 18, then what reasoning is there to view an 18 year old as a fully developed adult? There isn't. It's no different than saying 'well the legal age of consent is 14 so I don't see a problem.' You're still taking a legal definition and using that to rationalize your position.
So if I know that 18/19 year olds aren't fully developed adults (no matter how 'mature' they are), what reason would I have to be interested in pursuing them romantically if I'm 30 years old? I wouldn't...unless I view that lack of development as a positive, which in my opinion....is pretty creepy.
Human development is not some mystery. My GF has a PhD in this topic. It's been well studied across cultural divides.