Another common form of arthritis RA is an autoimmune disease. It happens when your body's immune system attacks body tissue. These attacks affect the synovial tissue in the joints, a fluid that nourishes the cartilage and lubricates the joints. Symptoms of arthritis usually develop over time, but may also occur suddenly. Arthritis is most common in adults over the age of 65, but it can also develop in children, adolescents and young adults. Arthritis is more common in women than men and overweight people.