Sponsored Content According to the American Academy of Orthopedic Surgeons (AAOS), more than 700,000 knee replacement operations performed each year in the United States. Although the majority of these operations are performed in people over the age of 65, a growing number of knee replacements are being done in younger patients. A study presented at the 2014 AAOS meeting found that the number of surgeries for patients 65 to 84 increased by 89 percent, while surgeries for patients 45 to 64 increased by 188 percent.