I grew up riding bikes, skiing down mountain slopes without wearing a helmet. Now you rarely see a biker who does not have a helmet on. And helments have become very common on the ski runs. That said, I don't wear a helmet very often while skiing or horse riding.
If I'm pushing the limits of my skills skiing, Then I will put a helmet on, But most days any more, I'm just out to enjoy the mountains, not see how fast I can fly down the mountain. The same is true for the horses. If I go do an endurance ride, where I will be around 100 strange horses, Yes I wear a helmet. A pack trip into the Uinta Wilderness, No I'm nore worried about a good hat to keep the sun off my face and the rain off my neck. I ride my horses a lot while hunting each fall. It's impossible to have a helmet on and try to sight thru a scope on a rifle.
My daughters and any of their friends ALWAYS wore a helmet. I found they didn't have the strength or skills to control the horses. Where I have sat out some pretty good bucks over the years and brought my horse under control and think I have some pretty good horsemanship skills to catch my horse before they even spook or blow up. Kids on the other hand, don't have those skills to control the horse and don't have the strength to hold on or the balance. And usually come off on the first buck. So the rule was that they always wear a helmet.
Every Monday pick up the paper and read about an ATV rider with a head injury from over the weekend. With and without helmets. But horse injuries are very assorted. Article last week about a woman who was heading back to Kentucky to compete on the national level. Giving her horse a "Quick ride" before loading it into the trailer. Horse stumbled and she came off and broke her back. She is a paraplegic now. Article was about her recovery and dealing with a life changing event. One a year ago there a very accomplished barrel racer was ridding her horse across the street. Horse slipped on the asphalt and went down, Breaking the riders neck and killing her. A good friend of mine, horse spooked and went off a cliff, he broke his back in 5 places, he recovered, but still has back pain and problems. Helmets didn't stop those accidents and didn't prevent the injuries. Accidents happen around horses and the best horsemen. And head injuries are a possibility, but not the only risk.
It's a life style with risk that we have chosen to embrace. Each of us has a different comfort level. Each of has has a different skill level and the quality of our horses varies, So we have to choose for ourself. But I made the rule that kids wear helmets. I felt the risk to kids was much higher and should be addressed. And my daughters and their friends never questioned it. While I finished tacking up their horses, The girls always rounded up their helmets.