Long answer in a few parts as AB wouldnt accept the original!
Total hip replacement (THR) is ball and socket, with a metal "ball" attached to a long stem that inserts into the femur (thight bone) and plastic "cup". There are several reasons why they *tend* to be done in the elderly, as Froggequene says. Firstly, most hip replacements are done for osteoarthritis. This is mainly a problem of the ageing body, unless you've previously had an injury that has damaged the joint surface, eg come off your motorbike and had a hip fracture into the joint. (There is another type of replacement that they do for the kind of hip fracture that old people get - however, this isn't a THR and if your bones are brittle enough to get this type of fracture, I certainly wouldn't run anywhere.)
Secondly, the stem is cemented into the shaft of the femur, and has a tendency to loosen after a while. This means that the stem wobbles about and if under extreme stress, eg running, the bone could crack. If very loose, the THR has to be taken out and replaced. You can get round this problem by using cementless THRs, where the stem is specially coated and the bone cells grow into the coating. Advantage is that they don't loosen. Disadvantage is that if anything happens to the THR, you'll never get it out again.