Canadjun has it right; generally, the only ones who wear helmets are
(a) epileptics whose seizures are common and severe. Knew a girl who was six foot three, and would keel over like a tree when she got hit with a grand mal; she averaged three a month -- and that was WITH medication. Hated the helmet, and it was hell keeping it on her.
(b) persons who have self-destructive tendencies. Worst I ever knew was a kid with autism who slapped himself as a self-stimulatory thing. We had a helmet on him that looked like a transparent welder's mask, because if we didn't, he'd slap himself, one handed, until the right side of his face puffed up and looked like the Elephant Man.
(c) retarded individuals with very poor impulse control. Knew a hyper little devil once who didn't MEAN any harm, but loved rough active games like soccer and football... and his coordination was a joke; he fell down right and left. We had to keep full skateboarding gear on him nearly at all times...