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. If you have things working for you, check out your horse with your tools. Get a saddle pad. Let the horse smell it. Rub it on the horse's back, legs, and belly.