This is a MUCH debated topic amongst riders.
Totally agree with kitten_val. That same rider came to mind as soon as I read the OP's post.
I am an adult who rides Western and I still wear a helmet. I do not consider myself experienced, however the experience level of a person has nothing to do with whether one should wear a helmet or not. The most "bombproof", experienced, Grand Prix dressage horse and rider can still have an accident.
To each his or her own. But I will be riding safer than sorry.