If severe brain development lasted until 30 most men would not be functional, thriving and supporting themselves by then. I have a son who is 40 and still thinks like a little boy. That's his fault. He rejected all sane, mature help
I think it is them not brain size. I think it is is self indulgence, personality disorders, selfishness, laziness or serious disabilities. Those with serious disabilities may be very kind..l don't put them in the same category as totally me centric able adults nearing 30.
We'd have a mess if most men truly did not grow up until 30. With the same brain size, most 30 year olds are not taking drugs, being unable to live without us, acting like defiant children and falling apart over normal demands. In fact, most have full time jobs and are self sustaining, many have families, many spent time in the military and/or college and many care a great deal about others beyond themselves, including our well being. They worry about us we get older, like we worry or worried about our older parents. But they only think of themselves usually, and that's not brain size. Many teens and younger with less developed brains care about others besides themselves. Sorry, the adult kids here think about us mostly in context as to how we can serve them. And everyone else. Like my son, they are selfish, especially toward those who love them the most, even BLAMING us for their problems.
Brain size? I think it's a more intensive problem...who they are.
Let's face the sad fact. Many of our kids lack normal empathy. Mine does. Brain growth doesn't change lack of empathy. Some time drugs cause the void of empathy. Some never had empathy before drugs. Some don't take drugs and lack empathy, like mine.
If they dont care what their scary behavior does to their own mothers, fathers and siblings...where is the empathy? Sad, sad, sad.