Regardless of the age of your horse, it is always important to get him desensitised. Although this might be far more difficult with older horses that have already cultivated hard to break habits and instincts, it is vital that he becomes accomplished to ensure the safety of any rider.
Desensitising is simply the process of slowly introducing to your horse foreign objects and items that might frighten him and help him get over those fears. Horses develop fears for various things over the course of their lifetime. They might develop these emotions through life experiences or certain primal instincts that were wired into their biological DNAs itself. Whatever the case is, it is possible for an experienced trainer to help your horse get over these fears and behave calmly and normally in the face of such situations.
The main objective behind desensitising your horses is to ensure that they do not get into a panicked state upon seeing a strange new object. Horses must be trained to keep their cool regardless of the situation to ensure the safety of both themselves and the rider. Nonetheless, it will do well to remember that there can always be exceptions to this rule, especially in particularly tense situations where the horse might forget its training and let its emotions overcome him.
Although desensitising might be hard work, the rewards are well worth the time and effort put into it. Make it a point to ascertain that every horse under your charge is desensitised. However, please bear in mind that like human beings, different horses also learn and pick things up and vastly different speeds. Certain horses might only require 3-4 tries before they learn that the saddle pad is of no threat to them and let you ride them. Yet, the exact same training routine might only start to seem effective after tens of tries on a different horse. It is important that you be patient, keep your cool and desensitise your horse according to its aptitude for learning.
On a final note, it is usually a good idea to give your horse a short runabout in the round yard to focus his attention and calm him down before you begin the job of training him. Never forget that horses are living creatures too and their moods and personality play a huge role in the behaviours.