Ya it's understandable if they were to take it slow and have the horses best interest in mind, but if you feel they were over doing it with this horse with the workout they did I can understand your frustration.
Sounds like you just know they are going to over do it and not really as upset with this one incident. Poor horse. I do think some horses can abe started this young. So much depends on the horse, the breed, the build, etc. At our barn there is a 4 yr Oldenburg and she has only done ground work, she says a trainer told her to wait to get on his back until is 5!
He hasn't really filled out yet and is all legs, but I think 5 is pretty long to wait.
By 5 most horses are ready to do whatever job they are bred for, but I guess some breeds are different.