25 is when people start taking you seriously. That's the age when it's expected that you have most of your shit together. Acting like and dressing like a teenager becomes socially unacceptable when you reach 25.
21 year olds are still boys in the eyes of many people. They can still be mistaken for 18-19yos and can get away with being irresponsible. Nobody actually trusts a 21 year old with stuff. Would you hire a 21yo lawyer or trust a 21yo doctor?