Thanks you all, for your kind thoughts. He was such a sweet mule.
Fifty-three IS remarkable, but my BO took wonderful care of him. He wasn't even kept in the pastures like the rest, the barn and surrounding area is where he hung out, with a 31 year-old QH mare. They were pals and would visit all of the horses in their different pastures through the fences. (The mare is still alive and healthy, giving riding lessons).
**I must not forget to thank the difficult horses, who made my life miserable, but who were better teachers than the well-behaved school horses who raised no problems.**