I agree with the Europeans! I bought a just started 4 year old this past December. I wouldnt look at 4 year olds that jumped around a course, though I saw one advertised as jumping 4ft courses. I have always believed a horse peaked between 10 and 15 years, if they are started right, thats when the best of the mind, body and experience seem to meet perfectly. You just dont see horses at the highest levels of competition that are much under 10 years old (dressage, eventing, showjumping - I dont know much about other sports). Even in a physically and mentally gruelling sport such as eventing, I know a number of upper level horses still going at 18 years old. A couple of GP showjumpers Ive seen are around that age as well.

My QH peaked this past summer at 17 years. We'll see what this summer holds (right now he barely resembles the sleek muscled show horse he was a few months ago, gotta love winter).