my best hiking partner

Duchess is my greatest companion on my hikes and scrambles. She loves mountains, and I was hoping that she would complete with me all Colorado fourteeners. I realized this summer that this will not happen. She refused to move on South Maroon (did not summit this peak), she barely made it back to the car, and could not walk for 2 days afterwards. Is it her age? or the difficulty of the mountain? Is it me pushing her too much. She was always ahead of me, including technical peaks like Little Bear, Crestone Peak and Needle. Well, I am happy now for the great hikes she did with me, and still hope for many more easier ones.