Demand for joint replacements, surgery once confined largely to patients well past retirement age, has been growing rapidly in recent years among a class of people doctors have dubbed “young actives’’ - those in the 45 to 64 age group who try to stay fit. Many still imagine the patient profile for artificial knees and hips, the most common joint replacement, as overweight older people seeking more comfort while cooking in the kitchen or watching their grandchildren’s soccer games or ballet recitals. But the patients seen by physicians today are just as likely to be athletic fiftysomethings who have run marathons and are looking to resume biking, skiing, or playing tennis.