Documentary from 1976. Features interview with Velma Johnston.
Remarks about trust beginning at 14:46 are off the mark. The ‘problem’ with older horses is not how long before you can trust them but how long before they trust you.
Here is a rule of thumb: Age ÷ 2
If you adopt a four year old horse, allow at least 2 years to earn his trust. The other option is to force yourself on him and break his spirit.