I know one person who is a successful hunter/jumper rider with a lot of experience. They have their 4 year old pony out in the rated show ring jumping 2'6" successfully. They made the comment that the ponies knees aren't closed yet.
Another successful hunter/jumper rider with a lot of experience has her 4 year old thoroughbred cross not jumping higher than 2' at the moment. She is working on gradual teaching without to much strain on the horse.
One of the riders though made the comment that 2'6" is the perfect height to teach a horse to jump because the striding and take off points are similar to what they would be once the jumps start getting higher. You have to ride anything lower than that in a different way and when it's lower the horse learns to see the jump and ride to the jump differently.
So my question is what are your opinions? How high is too high? Is 2'6" appropriate?