Growing up doesn't mean getting married, having brats and a white-picket fence home, working an office job, or becoming Stepford or Ward & June Cleaver.
What it does mean is that in your adult life you're able to handle your business, be a responsible individual, take care of yourself good and proper, and maintain some decorum and civility when dealing with your fellow man.
One can remain true-to-form with their philosophies, world views, ideals, sense of aesthetics,lifestyles, interests, fascinations, preoccupations, obsessions, indulgences, tastes and wants that may be on the fringe or subcultural and still be a grown-up.
Of course, if some things seem daft or tacky on an adult that's because they were already daft when younger.
Most of the people I know around my age still look interesting and have interesting lives and still have their bills and payments taken care of.
If you're true to yourself and genuine about the things you're into while remaining a productive, responsible person then squares be damned.