Projecting the wrong image
I once worked with an able colleague who looked even younger than his relatively tender years. He applied for countless promotions, and got lots of interviews but no offers. It wasn't difficult to see why. He was careless with his dress, his hair was all over the place, he couldn't resist making silly nervous jokes and generally came across like a soft lad. He forgot that a being a young and shambolic joker may win friends, and can work in the classroom, but it doesn't necessarily signal management and leadership potential.
What is the right thing to do?
Having seen it done, I've become convinced that you can help your career by paying attention to dress and grooming, projecting maturity and gravitas when it is required. A successful head summed it up for me once: "When I enter a room full of visiting adults I want them to know I'm the head," she said. "The governors need to see what they have spent their money on."