If we were to stipulate the absurd notion that under 25s cannot make rational decisions because the all-important pre-frontal cortex is not "fully developed"...

...how should that impact our society? 

What changes should we make? What age limits should we reset to make sure that these irrational young adults are not doing jobs they can't handle? 

Should we take the right to vote away from under 25s? Should we ban them from the military? Should we stop them from operating motor vehicles? Should we consider them working full-time child labor? Should we put their sex partners in prison because, if under 25s are children then clearly they can't consent to sex, right? 

I'm asking a serious question. What is the value of the "under 25s can't make rational decisions" conclusion that you've made? How should society act on that info, if in fact you really believe it?