I wear a helmet, have been a critical care nurse for 30 years, and used to teach the Motorcycle Safety Foundation classes, several points to make.
After 40 years of age, your brain/skull is a lot less able to take a blow than when you were younger, the suspensory system in the brain vault becomes much elastic.
If you fall over, stopped, and clip your head on a curb just right at any age, you're dead.
There are a million and one scenarios to play through about head injuries, IMHO, a lot of times motorcycle helmets just make it easier to have an open casket funeral, at speed, the injuries can be so massive, no reasonable amount of protection can suspend the laws of physics.
I won't ride without my helmet.