When you have an older horse, if you are looking to teach them tricks, it might be wise to find a trainer, or to get someone to help you properly train them. So, when you are looking to train your horse, it will not be such a hard task. Older horses have more fragile bones, may not be able to run as fast as they were able to in the past, may not be able to do certain tasks as they were when they were younger, and may have a harder time moving around all together, since their joints and muscles are older and more worn.I found some more information here.
So,when you are looking to teach your horse new tricks, hiring a person to help you out with the training may be in your best interest. Especially if your horse has never learned or done any tricks in the past, a professional helper would really ensure that they will be able to learn certain tricks more easily and more quickly. You will also need to ensure to give your older horse more rest when teaching it tricks. Do to its age, the bones and the body need more time for the horse to heal, and get back moving again.