I joined the Marine Corps at seventeen.
Delayed development of the ability to plan does not at all come from the brain not being developed.
It comes from not being made to shoulder responsibilities.
I have seen this proven over and over thousands of times, contrasting young officers and non commissioned officers with private first classes and lance corporals who are the same age.
Actually, it is not fully developed. Of course, it varies from person to person, regarding age, but tha skill I’m talking about is the ability to think about the long term consequences of decisions. Before age 25, humans tend to live more in the moment and make decisions based more on emotion.
It’s not that one is “stupid” until they are 25. It’s more like “a baby is not fully developed in the womb until a full 9 months is done. We have plenty of people running around that are fully developed and just like every body else, even though they were born at 8 months or even less. They’re completely normal. Their birth happened early is all.
Same with people that make major decisions before 25. Sometimes it is because others force them (parents make you go to college). Sometimes it is because they are taught, from a very early age, to prepare for the future. But the point at which that ability is fully developed is around age 25.