This presupposes that the brain development between 18 and 25 would still happen if 18-25 was still childhood. Do we have reason to believe that people can mature fully before being allowed to make decisions, that it's 100% about time and 0% about practice?

Note, debt is a huge problem for people don't have earning power yet. If 25 is the new 18, we should expect 25-year-olds to have the same financial instability as current 18-year-olds as they will be on similarly shaky ground in the workforce.