Because pool sides are steep and dogs cannot use ladders to get out of the pool, a dog can become trapped in the pool if he can not find an exit he can use, such as stairs. This can result in your older dog drowning. If you have an older dog that is used to having the run of his yard and doesn't understand that the swimming pool is not a good, safe place for him to take a dip, than you will need to teach your older dog to stay out of the pool.
If your older dog has never seen a swimming pool before, let's say you have just installed one or moved to a house with a pool, than you will need to establish the boundaries of the pool with your dog. This will mean supervising him and introducing him to the pool boundaries. Do not give your dog free access to the unfamiliar pool, in case your dog slips in or takes a dive on purpose!
You will then need to teach your dog not to approach pool boundaries, either by redirecting your dog, providing an alternative, or creating a negative association with the pool edge. Many pool owners keep their pool covered when not in use. Pool covers can also be dangerous, so you will want to train your older dog to stay away from the boundaries, both when water is open, and when the cover is on. You can use verbal commands, such as sit-stay to prevent access to the pool, or the leave it command if your older dog has an established response to this command. You can also direct your dog away from the pool with treats, or make the pool edge unpleasant, and even a little scary, to get your dog to move away from the pool edge.
You will need to supervise your dog while training him to stay away from, and out of the pool. If you and your family are using the pool, this may involve putting your dog in a separate location, like a crate, when the pool is in use. Make separation more a reward than banishment! Give your dog a rawhide bone or puzzle feeder to play with, so he doesn't feel he is missing out. Use treats to reward avoiding the pool boundaries, and “leave it” or “sit stay” commands. Barriers and noise makers can also create a negative association with the pool boundaries, which will cause your older dog to avoid the pool edge.