It isn't nonsense, it's true for men as well. The prefrontal cortex develops in the mid to late 20s and not just for women. This part of the brain is responsible for cognitive functions like planning and decision-making.I think in the short term any age limit is ok so long as everything is consentual and both parties are legal adults. This business of shaming men who go for younger women is mostly due to sour grapes imho-- and this business of girl's brains not being fully developed until they're in their 20s is nonsense. .
That said if you're thinking long term then an age gap is a bigger road block. A 50 year old can have a lot of short term fun with a 20 year old-- but in the long term? By the time she's 50 he'll be 80...
I don't care who consenting adults are having sex with (though seeking someone specifically because they're young does get a side eye out of me), when it comes to dating, yeah, I have a hard time understanding what someone has in common with someone decades younger who was in high school recently