I'm going to type the same thing that I always type on these threads.
I have worked for show horse trainers and I have rode and started ranch horses. For the most part in the western show world, yes, colts are started as two year olds. Working on ranches I have started colts as 3, 4, 5 and even older horses. I have lost count of the amount of colts I have started over the years, not saying that I am good at it, but I have started a lot. Here is what I have noticed.
Horses will stay sound or become crippled regardless of the year it was started.
I have seen two years old started, shown futurity, shown derby went on to aged events, packed non-pros then went well into their teens and twenties packing amatuers/novices and retire sound. Seen horses started as two years not stay sound and not make it to the show pen.
Also seen horses started in their 3,4, 5+ year go on to live a long life as a ranch horses still using them in their twenties teaching kids how to rope, cut and work cattle. Seen that same age group of horses be crippled and unuseable within a short amount of time.
The thing that people that only looking at the age of the colt don't take into consideration is the individual horse.
There are other factors; conformation, physical and mental maturity, the amount of riding done, the ground the horse is worked in and the difficulty of work being asked of the individual.
I DON'T LEAD 'EM AND FEED 'EM, I RIDE 'EM AND SLIDE 'EM.