Ultimately, I think there are industries where starting early is encouraged (or required) from a competition or financial standpoint. (Disclaimer: Generalizations below. There are, as always, outliers on either side.) Racing (TBs and standardbreds) is an example, but you’ll also find it in the horses that are tracked for the futurity classes, IME. In dressage, I see the YH candidates getting started earlier and earlier - same with some of the flashiest young stallions.
In some industries, this is really the reward for breeding for early January babies. Your “2 year old” is actually a 2 year old, as opposed to, say, the late may foal that is nearly six months behind the first-week-of-jan baby. For people involved in industries with age cut offs, this is one of the challenges. Even in dressage, doing a “4 yo test” at the first show of their 4yo year may be a January or February show, and they don’t turn four until June, if they were particularly late or bred in a location that rewards later foaling dates. Right or wrong, this is the impact of classes with age designations.
My 2yo (come April) post-weaning was out in a herd of seven fillies total. Four of the seven are now in positions where they are being started under saddle, and none of them have actually hit 24 months. I expect another will go very soon for training - if/when that happens, that means only two will not have had a rider prior to 24 months.
While studies show the benefit of working young horses, I’m cautious to use that as carte blanche to greenlight any-and-all work. There have always been questions I’ve had about these studies (what is the baseline horse they use, for one: I would love to see the young horses in/out of work that are kept stalled 23 hours a day vs those that live out in a herd 24/7 in/out of work, because I suspect this might show some key insights). Ultimately, teaching and training young horses isn’t a bad thing - but asking too much too soon is, and for me, it really can be a case-by-case scenario. Different horses develop at different rates (and in different ways). Would I personally have rides on a horse before 2… no. (Before 3…honestly, probably still no.) But would I turn away from a young horse who had 30 days of light under saddle work with a competent professional I trust not to rush training, especially if the horse seemed developmentally capable of tolerating the work? No. If nothing else, I can pick up the young horse, turn it out in a field again, and then bring it back in for periodic refreshers when I feel it’s suited to the task, so a horse has a solid foundation when they’re really ready to begin under-saddle work.