Given the age, I'm guessing Muscular Dystrophy - probably caused by a deficiency of Vitamin E and Selenium - but could also be genetic MS. You can purchase Vitamin E with Selenium in gelatin capsules at your local health food store. Pierce the capsule with a pin and squirt the oily contents directly in the bird's beak once a day. Make sure this bird is receiving a vitamin-enriched, nutritionally balanced feed and make sure your feed is fresh. Vitamin E loses it's effectiveness rapidly if feed becomes stale. If you don't see improvement over a week or two, the problem may be genetic MS or another disease that attacks the neurological system (like Marek's).