Accidents are just that...
I was 17 years old the year I began working at a horse farm in Ocala, FL training horses. I had grown up around horses and owned horses since I'd been 12, and never owned or wore a helmet.
This farm had a policy that any rider under 18 years of age was required to wear one, so in order to keep my employment there, I went out and bought one. (it was a Troxel Grand Prix, incidentally)
One day, I was riding a young, 3-year old Andalusian stud colt in a grass arena, just putting him through his paces and having a nice, relaxing ride. He was calm, quiet and was doing nothing wrong. We began to ride a circle at the counter-canter, and being as young and gawky as he was, he tripped over his own feet and literally did a somersault, his back end rolling over the front and throwing me to the ground with force. I hit the left side of my head, right above my temple. It happened very quickly; I don't even remember being able to put out my arms to brace my fall.
The barn owner was long ways away working another horse in a round pen and claimed to have heard the "crack" of my helmet hitting the ground. He looked up and saw the stud colt running free and me lying on the ground, unconscious.
I stayed out for several minutes, and even after I "came to", I went in and out of consciousness for quite a few hours afterwards. I had short-term memory loss, my head hurt for many weeks; I was sensitive to light, loud noises and couldn't bend down to pick up something from the ground without blacking out. My eye turned black and blue and I had some brusing on my face and cheekbone. But I know without a doubt that if I hadn't been wearing that helmet, I would not have been alive to talk about it.
Because of that incident, long after I turned 18 I continued wearing helmets and do to this day now almost 10 years later. I am a professional riding instructor and trainer, and all my students wear helmets at all times. My advice? Make it a way of life no matter hold old you are or how good of a rider you are. I have fallen off a horse 63 times to date during my equine career, and I love being alive and able to share the stories!