We used to d high jump challenges with our two kelpies.they passed at 16 and 17 and were hardly ever lame, if they were it was usually grass seeds........BUT.........they were very fit dogs and jumped in and out of yards, on to our horses backs , onto railings, on the utes forever. We never forced them and they loved it. If they refused a jump we left it at that. I think it depends on breed and soundness. Our kelpies could also work for hours on end and were always ready to go. I still miss all that energy