Yes. The Triple Crown (Kentucky Derby, Preakness Stakes, Belmont Stakes) are for three year old horses only. I could say much about the horse racing industry. And much of it would not be positive but put it this way, $$$$$$$$. I am no fan.
There have always been very hotly contested debates about when a horse should be broken to ride. Some vets say it's ok at 3 while others say wait until 4. Some old school trainers broke horses at 2-3 years. Some trainers still do. Most of the trainers we know, who have adopted more humane training methods, will not put a rider to horse until the horse is 4 years of age.
My lady friend here worked with horses out west many years ago with Mia Lykke Nielsen (from Denmark). They're still close friends. Both train horses the same way. Mia developed a training method that is very loving, humane and the least stressful for horses.
More and more people around the world are using her methods. Many folks bring her so called "problem" horses they "claim" cannot be ridden. There isn't a single horse that she hasn't been able to train to be ridden.
Mia's site:
https://www.whenhorseschoose.com/
Her YouTube channel:
https://www.youtube.com/channel/UCp57hYBvEB_D5wZRLhvbElQ
The videos she posts demonstrate her methods.
She is a "true" Horse Whisperer and very much in demand.
The Horse Whisperer - The Horse Rider's Journal That article is from 2016 and describes her methods in detail.