Age can be factors in training as well. The young horse can be developed through growing years and groomed into an excellent performer. But the second hand aged horse over the age of 8 needs a more specialized approach. A green broken 15 year old horse who has a good walk, fast canter and choppy trot needs something special. Narrowing down possibilities due to the fact that she doesn't have time to develop her trot for two years (a trot she's been comfortable with for 15 years...not happening) so I need to find her a place with what she has now. Good walk with reluctance to break into her choppy trot makes for a great hobby trail horse. Her excellent stop and fast canter are great for barrel racing. At those times she would just need to learn movements while building muscle for those things without needing to take years relearning her own gaits.
Using this process is also good for finding injury. Horses are prey animals, so they hide injury and pain to prevent predators from singling them out as weak. So when you see obvious pain and lameness in a horse it is then beyond what he can reasonably handle and more serious. The earlier you can spot lameness and injury the faster and more likely he will recover completely.
Being able to create a specialized learning environment for every horse within a couple hours session is a true art needing apprenticeship to truly learn. Much can be learned in the classroom and theory techniques can be taught in a group. But the feel, balance and listening to the horse must be learned with someone who has been doing it for years. I do not claim to be a master; I have not yet finished a horse to relative perfection, But I have studied with them. I have completed my apprenticeships and have moved on to making my own experiences; a journeyman.