Most probable cause at this age is osteoarthritis... It's a degenerative bone disease...There’s no cure for osteoarthritis as yet, but there are a number of treatments that can help ease symptoms and reduce the chances of her arthritis becoming worse...like analgesics, NSAIDs,intraosseous steroid injection and hyaluronic acid injections... Treatment depends upon the stage...I would suggest you to consult nearest orthopedic surgeon they will do the needful.