"Adolescent" is a straight pull from Latin it's literal meaning means being in the process of growing up and maturing. Adolescent is not still a child, adolescent is a child recognized as being an in-progress-to-adulthood.

"Adolescent" 25 year olds is the problem, 25 year olds still in the process of becoming adults because of progressive infantilization where parents seeing their children aren't adults are delaying further and further treating their children like they're becoming adults ... is the problem.

You're looking for a new definition but you shouldn't be.

Maybe we shouldn't send our 13 year olds out into the wilderness for a week with only a knife and a water bottle like many societies have done... but maybe our 26 year olds need to be embarrassed when they can't take care of themselves or be responsible for their own decisions.

The process of lengthening childhood into the mid 20s is the problem and new definitions isn't the solution.