This is a common question, and I am not sure I have a great answer.  Running requires impact loading of the lower extremities, including the knees.  For some people that have knee malalignment, meniscus deficiency, cartilage erosions or genetic defects in cartilage, knee instability, and those that are carrying too much weight, then the repetitive loading of the knee can accelerate arthritis and degenerative changes.  In addition, there is some evidence that running styles can affect the resultant load on the knee.   Newer materials are now available to make running shoes better as absorbing load and can minimize the load on the knee.  For those that have a biomechanically efficient running style and do not have predisposing risk factors for knee arthritis can likely run well into the latter years of their life.