I have seen a lot long yearlings started and when done correctly it is no worse then what I have seen on horses who where started as 3+ years old.
This is simply do to the fact that starting them that young the trainer is not felling pushed to get them ready. They spend A LOT more time just walking the horse and pushing the horses body around. This builds musle in the horse gets them fit gets them use to carrying weight. By the time they are into their 2 yo year and ready to get more into real work they are more fit and so on to do the work proplery. I would much rather have a horse started say in Dec of their yearling year and started slow like most good trainers will do then stay wait until they are well into their 2 yo year and then you have to push them a bit more to catch up. The people I see waithing until the horse is older just end up thinking the horse is ready to be pushed faster and at the end of the day they stay no more sound then the one started correctly as a long yearling early 2yo.