Having a ring around the pupil is common among elderly people, who often spot the condition in a mirror. The whitish arc you see is associated with fat (lipid) deposits. Sometimes the condition is associated with cholesterol levels in the body.
Discoloration caused by arcus senilis does not decrease vision or harm the eye. However, most eye care physicians feel that people with arcus senilis under the age of 40 should have their blood tested to check for elevated cholesterol levels or other lipid disorders. Research shows that younger male patients who have rings around their pupils tend to have an increased risk of coronary artery or cardiovascular disease.
Catania, Louis J. Primary Care of the Anterior Segment, Second Edition. Appleton & Lange, 1995.