In the performance horse world and with many breeds, 3 to 4 years old should be an adequate age to start a horse under saddle. In the racing breeds we start a little earlier (around 2 years of age) and the data doesn’t show that starting these horses early is a problem. But, generally, 3 to 4 years old is a good age and the horse should be plenty mature skeletally for some riding.
Horse aren’t completely physically mature at this point and are still growing, and certainly a 3-year-old Warmblood will look different from a 6-year-old Warmblood. You have to play it by ear and…
Setting aside everything else about this statement, there’s some lag time there.
Assuming a maximum rate of horse-sexing is initiated today with no organizational delay:
https://www.petmd.com/horse/pregnancy-horses
Then you’ve got time for a foal to grow and then to train it.
https://thehorse.com/115040/when-is-a-young-horse-ready-to-ride/