We've all seen the evidence that shows that horses aren't completely physically mature until they're 5-6 years old. Their growth plates aren't fused and their back is still developing until 5-6 years old. So this should mean that horses aren't ready for riding until then, right??
Overall, I believe that horses should be assessed on a case-by-case basis to determine an appropriate age for saddle starting. While I wouldn't recommend high intensity exercise for young horses (under 6 years old), I believe that a certain degree of exercise is essential for healthy development of the musculoskeletal system of horses.
Let me begin by making a comparison...
Human physical growth doesn't finish until around 18-20 years old. This is when the average person has all epiphyseal bone plates closed. Does this mean that we should restrict children and teens from participating in sports because it will stress their young joints? Of course not! Instead, physicians recommend a high level of physical activity because it helps improve bone density and joint health. Now let's think about how this relates to horses...
Foals in the wild are designed and expected to keep up with their herd within hours of birth at a full gallop if necessary! Young horses in their natural environment spend their days moving constantly. They travel an average of 30 miles per day, they spar and "play-fight" with other young horses daily, and they are expected to climb mountains and jump obstacles.
In domestic situations, foal management more often takes a very different appearance. Young horses are often confined to a limited pen or paddock with significantly less interaction with other horses. Horses in stalls or even larger paddocks simply do not travel nearly the distance that their wild counterparts do. Horses then begin training at some point between 2-6 years old.
Research has shown that training while horses are still physically developing actually HELPS the development of strong and sound bones, joints, and cartilage. This is because the musculoskeletal system, as it grows, is accommodating to the level of exercise that it expects throughout life. When a horse sees nothing but a stall and moderate turnout until they reach maturity, their bodies grow to expect that light workload. However, horses who see consistent turnout and moderate exercise including work under saddle develop stronger bones, joints, and cartilage in both their legs and their spinal column. In fact, one study showed that earlier exercise led to less incidence of injury and lameness later in their competitive career.
This is when people typically object and bring up race horses, and other performance horses. But thoroughbreds are often started under saddle at 18 months old and raced at 2 years....then they have to be retired at 5 or 6!! This must mean that the strain on their young joints caused their lameness!
Of course there are extreme circumstances. Most commonly, when horse owners become competitive, they don't necessarily put the well-being of their horses first. After all, there is money to be won and these horses are expensive! This is where trainers are galloping Thoroughbreds on the track as hard as possible when they're only 2 years old. This is where performance horse trainers are getting dizzy spins and 10 foot slide stops on 2 years olds so they can be sold as futurity horses and turn profit sooner than later. There are extremes to every spectrum. It's not necessarily a sustainable practice to keep a horse in a box stall 20 hours per day and intensely exercised during the remaining time. This is where we see the breakdown of horses early in their lives.
So what is the appropriate balance of exercise for young horses?!
It depends! (Yes those two magical words that you love to hate...)
Let's think about a few different scenarios...
Would it make a difference if I told you this horse is a champion reiner who is ridden by a 6'+ man?
Does it make a difference if the rider is balanced or unbalanced?
How about a saddle? Is it appropriate for a two year old horse to carry a 40lb saddle without a rider?
What about letting a small child sit on a yearling? Is that damaging?
Does it make a difference if the horse is exercised in circles or straight lines?
There are so many factors that contribute to how best to maintain the health and soundness of a horse. Simply using a hard and fast age rule is not enough.
So how do I determine whether a horse is ready to be started under saddle?
I look at the build of the horse. Some horses are broader than others. Some horses are naturally more balanced than others. If a horse is apparently broad and balanced, I don't mind putting my 120lb frame on him to let him carry me on light trail rides and doing light-moderate exercise where I'm working his mind more than his body. There is so much you can do to soften a horse and build their responsiveness without riding them into the ground. However, if a horse is quite narrow, lacks good musculature, and appears unbalanced, I might wait until he matures a bit before I put myself on him.
Regardless of what age I sit on a horse for the first time, I try to allow all my horses (not just the young ones) as much space to move and play as possible. I also believe it is never too young to play with your horse on the ground. There is an incredible amount of bonding, softening, and preparation you can do beginning on Day 1 of a horse's life!
So all in all, I've started horses as young as 18 months and as old as 25 years. But to me, starting doesn't mean we're jumping right into high intensity workouts 6 days per week. Use your best judgment and really consider the individual scenario when deciding to ride your young horse for the first time or before criticizing others!