Personally I think it depends on the riders experience, age, strength, and patience.
The age of the horse almost doesnt matter I find, I started riding my horse when she was two years old, I was only 12. She was incredible, within a year we won a highpoint championship. She was a quarterhorse. So I do agree that quarterhorses and paints are very easy going and good to work with, no matter what age and experience. Although there are they few that you have to watch out for
I always find thourogbreds a little trickier to ride, they are very forward and you need a lot of patience to ride them.
Just to switch it up a little I find warmbloods to be good jumpers and a little easier then thourogbreds.
Maybe even a morgan
I love morgans. They are great horses to have, and have good endurance.
For an age range I would say 8 - 12yo. Trained, but still has potentail to go further.