A huge controversy in the equine world is the practice of starting equines at a young age. You see race horses,, show horses, reining and cutting horses and other performance equine being put under saddle as early as 2 years old. This practice is in essence abuse. No equine is developed enough before the age of 5 to work under such stress and weight. The developmental information below supports the practice of waiting until an equine is 5 to 6 years old to ride. If you want to start riding an equine immediately, we suggest you purchase an equine of riding age that has not been abused (ridden too soon). At least 5 years old for a horse and at least 6 for a Mammoth donkey.

Prior to this age the horse, mule or donkey is like a young human toddler and teen. Their bones, tendons, muscles and growth plates are still developing. Asking an equine that is just learning to walk, trot and run in balance to perform under weight can literally destroy the animal. Consider that the number one medical complaint in the equine industry is lameness orthopedic injuries. Futurities and money making equine practices have really destroyed the longevity and health of the equine today. I want all my equine to be able to go for trail rides into their twenties. Pushing too soon ruins them forever. Remember all the preparation should be completed for riding before you swing your leg over their back:

Halter
Pick up feet
Blanket
Saddle – Just get use to the girth etc.
Trailer loading
Back up
Whoa
Walk
Trot
Canter
Confidence building tools and more

The Schedule of Growth Plate Conversion to Bone in horses:

Mammoth donkeys take 1 year longer in physical, emotional and mental growth than the horse

The process of growth plates converting to bone goes from the bottom of the animal up.

The growth plate at the top of the coffin bone, the most distal bone of the limb, is fused at birth. This means it gets no taller after birth but does get larger around, through another mechanism. After that the growth plates fuse as follows:

Short pastern - top and bottom between birth and 6 months.
Long pastern - top and bottom between 6 months and one year.
Cannon bone - top and bottom between 8 months and 1.5 years
Small bones of the knee - top and bottom of each, between 1.5 and 2.5 years
Bottom of radius-ulna - between 2 and 2.5 years
Weight-bearing portion of glenoid notch at top of radius - between 2.5 and 3 years
Humerus - top and bottom, between 3 and 3.5 years
Scapula - glenoid or bottom (weight-bearing) portion – between 3.5 and 4 years
Hindlimb - lower portions same as forelimb
Hock - this joint is "late" for as low down as it is; growth plates on the tibial and fibular tarsals don't fuse until the animal is four (so the hocks are a known "weak point" - even the 18th-century literature warns against driving young horses in plow or other deep or sticky footing, or jumping them up into a heavy load, for danger of spraining their hocks).
Tibia - top and bottom, between 3 and 3.5 years
Femur - bottom, between 3 and 3.5 years; neck, between 2.5 and 3 years; major and 3rd trochanters, between 2.5 and 3 years
Pelvis - growth plates on the points of hip, peak of croup (tubera sacrale), and points of buttock (tuber ischii), between 3 and 4 years.
The vertebral column-A normal horse has 32 vertebrae between the back of the skull and the root of the dock, and there are several growth plates on each one, the most important of which is the one capping the centrum. These do not fuse until the horse is at least 5 ½ years old. The taller the horse and the longer its neck, the later the last fusions will occur. Fusions in male horses generally take up to an additional 6 months.

For more information see http://www.webertrainingstables.com/ridingyoung.cfm