My mare I rode quite alot, and fairly hard I admit. She loved the long rides and long trails. She loved to get up and go. Even when she foundered she wanted to go, and bless her, she tried. (no I did not ride her when she'd foundered) She wanted to be able to keep up with the other horses in the pasture but couldnt. She was around 20-21 when I made the decision to put her down. Had she'd not foundered, she'd still be a get up and go horse today Im sure. (she would be 23). But I guess my point is, it doesn't depend on the age when you retire your horse, but it's ability and soundness. If your horse is still fit and wants to work, then don't retire her until she just cant work anymore.
My girl was also a paso fino mind you.
Good strong hardy (hearty?) little guys them Pasos.