The horse is going to tell you. By this we mean that the horse will have been exposed to enough situations that this will just be another exercise. Our belief is that you do things on the horse's time. Your horse must have a good posture, be calm and relaxed. Don't put a time limit on this.
Some horses are ready at two, some at four.
There's nothing that says you definitely have to start a horse and be on them by a certain age. I remember someone asking Buck Brannaman a question at one of his clinics about how long they should work with their horse, because it just wasn't responding to a one rein stop. What he meant by this was don't stop until you get to a point where you have made some progress. Although, you need to determine what progress is, it may be standing still for two seconds. There is some risk involved in horse training for both you and the horse.