It's how ignorant people have become. I train my lesson barns yearlings and babies. I've never ridden the two youngest (coming 3 y/o and coming 2 y/o, I think) - just ground work, putting bridles on, light saddles - ect. It's deifnantly not okay.
No. 1 sin against horses: pushing them too fast, too hard. A shocking number of people are still in the 1800's in regard to horses, considering them things, work tools, to be used like a shovel or a hoe (don't call my mare no hoe!), and if they break under the stress of the work, they are thrown away in favor of new ones. And you will even see this from people who claim "natural horsemanship" without having a clue as to what that even is.
When I was looking for horse years back some guy sent me pics of his yearling with his 2(!) daughters (one was almost as big as me) sitting on top to show me how "calm" the horse was. When I picked a different horse (my qh) he kept whining me over email that "I knew it's not going to go thru, I just knew it...."