A horse telling you when he is ready. For me nobody can tell a horse when he is done. Great horses that love her job can show in the grand prix until a very old age. At the same time courageous horses with a lot of qualities sometimes don't know what they can't do. I learned that pushing a young horse to fast in the end it's not the best for the horse. In the end there is no right or wrong answer to your question. The horses tell you.
My trainer has a 4 year old holsteiner that he's training in dressage and said by the time he is 8 he will be showing in the Grand Prix