There is no such thing as an early or late bloomer in terms of bone development, which I think is one of the major keys to a sound, long life. The other is careful conditioning.
From birth to 3 mine get handled every which way–tying, leading, ponying, trailering, walking over tarps, going to shows, etc. and plenty of out on the trails with adult horses, or out in a huge field with hills to run.
At 3 or earlier start making a saddle and bridle just part of life. Maybe late 3’s start putting weight on back and sliding off.
4 very slowly start riding. Since all of the prelim. work has been done, nothing is usually an issue. Light, easy arena work and trails. It’s not about pushing, but just letting them experience everything. I find they’re usually working on focusing on where to put their feet, so don’t have the attention to do other things.
I find 5 is the year where they know where to put their feet and feel like they are super cool now, so start testing. Slowly start asking more in terms of staying together. This is where the conditioning steps up, and it’s not about pushing in the arena, but longer times, and preferably a lot of trail with hills.
6/7 they start really building the muscling and conditioning, and the last of the bones close in the spine, so it’s the START of the collection and really bringing them together.
I think it is a concern that we have “naturally talented horses that can do the work” at 5/6. They might be “doing” it, but correct dressage requires a lot of training and conditioning, and it takes minimally 2/3 years of really focused conditioning to get the soft tissue as well as bony tissue where they are truly doing it correctly and preparing for a lifetime of soundness.
The traditional Lippizzaners don’t even ride them until about 6, and they are staying sound and working at a very high level well into their 30’s. I think that’s a lesson to learn.