Let me say straight out that I do not train young horses, so I am speaking from personal philosophy, but not much experience: I think it has more to do with a horse’s level of training than age. With one of my horses, who started out as a hunter, I didn’t do any substantial sitting in my dressage work until we were solid first level and starting to do second level movements. Prior to that, it was just a few minutes at a time to get him used to the idea. Until he learned to really move through his back like a dressage horse, it was pretty obvious that sitting was harmful, not helpful to the quality of the gaits.
IMO the recent change to allow sitting at first level brought the dressage tests into line with practical training. Some first level horses (and riders) are ready for it, some aren’t. I think that it is appropriate to start doing substantial amounts of work in the sitting trot as soon (or as late) as the horse is able to give you a supple place to sit, and the sitting can help to improve his gaits. The age where this occurs will depend on the horse’s conformation, the rider’s skill, and the amount of training/muscling. A horse that is built really uphill and moves through his back easily will enable a rider to sit earlier than, say, your typical arab or quarter-horse. Even though a quarter horse may have a smooth and seemingly sittable trot, sitting it early on will just drive them more downhill and will not help lift the shoulders.
I would think that many of the teams preparing for the 4 year old young horse tests are at that stage - but many most of our regular 4 year olds are not.
Yes. I do some sitting before the horse is actually at that stage. But sitting before the horse is at that stage would be just for short amounts of time in order to get him used to new things, and is not for helping to train a quality gait.
As I said, I am not a young horse trainer. I am throwing these ideas out there more to hear what others think of them rather than to give advice.