Joint-replacement patients these days are younger and more active than ever before. More than half of all hip-replacement surgeries performed this year are expected to be on people under 65, with the same percentage projected for knee replacements by 2016. The fastest-growing group is patients 46 to 64, according to the American Academy of Orthopaedic Surgery. Many active middle-agers are wearing out their joints with marathons, triathlons, basketball and tennis and suffering osteoarthritis years earlier than previous generations. They're also determined to stay active for many more years and not let pain or disability make them sedentary. To accommodate them, implant makers are working to build joints with longer-wearing materials, and surgeons are offering more options like partial knee replacements, hip resurfacing and minimally invasive procedures.