I`ve started a few youngsters in endurance riding, and this is my schedule.
For the first 3 years, they run free on a big mountain pasture. They learn how to balance themselves racing up and down uneven ground. They play with the herd and learn to be horses. They also get plenty of ground lessons, how to tie, be groomed, feet trimmed, etc.
At 3 the first "riding" lessons start. Getting used to all the equipment, advanced ground lessons, walking over and around things, being ponied into new surroundings, ground driving, lunging with purpose (not just running in circles).
At 3.5 I`ll actually get on their backs. I`ve got a great dressage trainer/instructor who understands about endurance riders. He gets the youngster for about 6 weeks. During this time they are ridden about 1 hour per day, in the ring and in the field when ready, 5-6 days per week. I`ll ride and train along side of him. Then baby comes home and continues weekly lessons with the trainer.
Between 3.5 and 4, there is usually a gap of 2 months when all the horses are just running around on a cold snowy mountain. Dec and Jan are too cold for *me*.
At 4, more or less, I`ll start with light trail work. One or two days a week, the youngster will get walking and trotting on the mountains. It might be a short ride of an hour, or a longer ride of 2-3 hours. But it is very slow. Training is getting used to the surroundings, trail manners, carrying through the dressage training onto the trails. There will be one day a week of a dressage lesson. All the basics, none of the "fancy" high level stuff.
At 4.5 or so, depending on the mental attitude of the horse, I`ll take them into a slow limited distance ride. I might do 2 rides during the 4th year. Training here is "listen to me, not the other horses."
At 5, I`ll evaluate the horse and decide if they are ready for heavier training. They still get a day a week of dressage training. A bit more advanced - up to training level:-) dressage. They still get a day or two of trail riding, but now, adding in just a bit of speed or longer distances. And all get turn out on the mountain. I`d start with a slow limited distance ride, and move up to the same speed or slower 50.
At 6, I`ll evaluate where I want to be and decide if the horse is ready to get me there. And train accordingly.
Each horse is very individual, and as the trainer, you must customize your program to fit the horse.
If I were starting an older horse, I would probably start with the 4 year old program, and move up as the horse`s condition allowed.
With Dragon, we went through the 3 year old program twice. Once at 3 and again when he hit 4. His little brain wasn`t quite up to the program. Physically, he`s ready to go. So now, going into year 5, I`ll start the 4 year old program. The brains of these guys are just as important as their bodies.
The horse will tell you when they are ready to move up a level. On physical conditioning, the horse`s bones are not ready for hard training until they are 5. Arabs keep growing till they are 7 or so. You can walk forever and not hurt them. You can start with some concussion and watch carefully, this would be trotting on hard surfaces (like my rocky trails, or hard packed roads). But at the first sign of lameness, even if only minor, back off. If you use a heart rate monitor, you`ll know pretty quick when you`ve conditioned the heart, and can move up.
One set of numbers I`ve heard:
It takes 5 years to condition the bones.
It takes 3 years to condition the hard tissues (tendons and such).
It takes 3 months to condition the heart.
It takes 3 weeks to condition the sweat. (If you are changing from a dry to wet climate.)
With the youngsters, slow is much better than rushing anything.