I have had four of my horses die of old age, three of the four had been retired for YEARS before they passed away. Sure, I could have found them other homes and had four nice pretty new playthings in their stalls, but that thought never would have crossed my mind. Why? Because I valued them, their loyalty, their willingness to try for me even when their bodies were telling them they couldn't. I would have never cast them aside just so I could have something "younger and more fun" to play with. Shame on your mother, OP. And shame on you for just saying "oh well", and not doing all you can to give that older pony of yours a decent retirement after all her years of doing whatever you asked.
"Keep a leg on each side and your mind in the middle"