I don't wanna grow up

If the brain hasn't finished development until the mid/late twenties, but both men and women are sexually fertile well before then, why isn't the experience of marriage and child rearing supposed to be a part of forming the permanent brain structure?

Sure, they'll make mistakes and so forth, but the things they'll learn, the behavioral adaptations they'll make, all while in their physical prime, mean that they will have been conformed to their life circumstances in so much more natural a way than people who try to "learn" these things after their brains have already settled.

Think about it. If you've never experienced something before, and your brain has "finished" developing, how easily will you be able to start forming new folds, learning completely new patterns, compared to when you were younger?

How is the energy level and general wellness capacity of a 30 year-old versus the same person at 20?

Why does our society want to ensure that these lessons, these bonds, these connections, don't occur when they have the greatest chance of making a permanent difference in the behavior of the individual?

No kids/marriage by 20 was seen as a failure by past generations for a reason, and scientific research has only come along later to affirm why such views were correct, even before we had the hard data to support them.

There is almost nothing good about a world built according to human ideals, but we keep trying, thinking that maybe this time the power of our ideals will be enough to reshape the reality we exist in.

It won't be, and we'll instead invite only tragedy as a consequence.