Within these contents you will find plenty of information about flexibility, strength, and proper nutrition. Don't blame age. Put the blame where it belongs. My quads are strong, but I feel like my hamstrings are going to snap. This is a consequence of muscle imbalance.
People sometimes fail to rehabilitate injured muscles.
They don't stretch the injured muscle or strengthen the muscle. The previously injured muscle will imperceptibly deteriorate and be ill equipped to handle the demands of sport. For instance, an athlete will train the quads but desire not to strain the previously injured hamstring muscle. This will make the ultimate problem worse. It is not for nothing that people describe rehab as difficult and painful.