You can start jumping around ages 5-6. Racing career ends for flat racers. Fine for brood. Start Dressage, which is too complex for younger horses. More complex Saddleseat moves.
Steeplechasers start racing.
Fine for brood. Polo horses, jumpers, and gaming horses are usually at their prime. Steeplechasers are retired. Dressage horses and world famous Olympic horses all compete their best at this age. Good for any riding level depending on the horse.