I think that even though the potential new horse is 19 y/o, he can still learn how to be mounted properly. It may take a little longer to teach him, but he can still learn for sure.
I liked the posts about flexing each side and then flexing once you get into the saddle, so you don't take off and ride right away. The only extra tidbit that has worked for me in the past is to make their feet move if they try and walk away. For example: I will slightly flex my left rein and have the horses head to the left so that when I mount and they want to move, I can flex them around and their hindquarters will go the opposite way and avoid stepping on me. Once they have moved their feet, I will take my foot out of the stirrup and MAKE them WORK from the ground. I will have them yield their hindquarters or lunge a few circles and change directions. Once they have calmed down, I will repeat the process. They learn that it is just easier to stand still and moving means more work. Good luck!!