ycbm
Overwhelmed
I've known a handful of horses who were overcooked by being over ridden when young, but really very few, and in all of those cases I would say that they never really had the temperament to be doing the job they were intended for without very slow training anyway.
But I have known a lot backed young who didn't have the mental resilience at 3 to be corrected (however sympathetically) without a small amount of dramatics, which then frightens their rider and a downward spiral begins. And/or their rider-owners didn't know them well enough through other types of interaction first to be able to have positive 'discussions' intuitively once on board.
That can happen at any backing age of course, but it is less likely if the owner has spent time doing other types of training first.
On the physical side though, my very good physio is convinced that getting of a three year old once a week for 10-15 mins walking is beneficial.
Great post DD.