Knee replacement is possible at the age of 40, as long as there are indications for knee replacement, such as severe osteoarthritis or severe loss of knee function due to rheumatoid arthritis or ankylosing spondylitis. At 40 years of age, due to relatively good cardiopulmonary function, knee replacement is safer than in older patients, and recovery is faster and more advantageous in terms of preserving joint function, so as long as there are indications, knee replacement can be done at 40 years of age.