I haven't had a hip replacement myself, but a friend of mine who is a high school mountain bike team coach has had both hips replaced. She was about 65 when she had them replaced and went right back to riding within just a few months of having the surgery. I was surprised at how quickly she was able to recover and get back on the bike. As with all surgeries, it probably matters a lot how fit you are to start with, and how conscientious you are about following the physical therapy instructions through to their conclusion.