Keratoconus is a progressive eye condition that alters the shape of the cornea, which is the protective clear layer on the front of your eye. The condition causes the cornea to become thinner and thinner and gradually protrude in the shape of a cone. Keratoconus can develop at any age, but it's most often diagnosed in teens and young adults. It tends to affect both eyes, but it doesn't always affect them equally.