"Gaited horses mature more slowly than quarter horses. That's why you can
start a quarter horse as early as 18 months to 2 years, but you should avoid
starting a gaited horse that early, or at the very least, make sure the young gaited
horse has limited rides."
This article blows that argument out of the water! Author Deb Bennett Ph.D., says
“There is no such thing [as a] slow-maturing breed. The Quarter Horse is not an ‘early maturing’ breed – and neither is the Arabian a ‘slow maturing’ breed. As far as their skeletons go, they are the same. This information comes, I know, as a shock to many people who think starting their colt or filly under saddle at age two is what they ought to be doing."
She makes a compelling argument that ALL horses continue their skeletal growth until about six years of age. Therefore, while she advocates basic groundwork from early on, she recommends waiting to actually ride the horse until he is at least four years old.
Here is the full article if you'd like to learn more.