Why do people have such issues with older horses?

My Horses is 17 , she is completely sound, ( More so then my coach's 9 year old , who is Lame Often) My Horse still competes , Jumps 3'3ft and Wins often. I do Things for her Because of her age, Like supplements and Long warm ups and cool down with cooler. But she is perfectly healthy. If you take good care of your horses they can stay healthy a long time. and sometimes Young horses, like my coach's , Even when taken good care of still have trouble. Age isn't a very good way to diagnose a Problem.