You do know that some hairstyle can either make you look old or young. I don't believe that guys always stick to one hairstyle. As you grow, your preferences change. If the guy doesn't care about his appearance much, then they would stick to one hairstyle for his entire life but not really cus some gone be bald
I change my hairstyle all the time :P A year ago I had long hair , sides trimmed and man bun (sometimes). After that I had my hair like Boyka and now my hair is like it is in my picture. Anyway , I doubt it has anything to do with confidence issues. It has more to do with guys being comfortable with their hairstyles. If it suits them they stay with that particular haircut.