As eyes age, eyelid muscles weaken, and skin becomes thinner and more flaccid. This can cause the upper lid to droop or the lower lid to sag. Eyelashes and eyebrows may lose their lushness and thin out considerably.
Tear production also drops off, and the oily film that tears provide decreases as lubricating glands in the conjunctiva (transparent membrane lining the inner surface of the eyelids) fail. These changes can lead to a buildup of mucus, resulting in stickiness, or make the cornea (curved, transparent dome of tissue at the front of the eye) dry, causing irritation or an uncomfortable, gritty sensation in the eye.
The conjunctiva turns thinner and more fragile with age and takes on a yellowish tinge from an increase in elastic fibers. The sclera also assumes a yellow hue from a collection of lipid, or fat, deposits. Calcium may deposit in the sclera (white of the eye), leading to patches of grayish translucency. The exposed conjunctiva between the lids begins to degenerate, and the cornea can develop an opaque white ring around its edge.
With time, the crystalline lens hardens and loses its elasticity. This makes it more difficult to focus on near objects, a common condition called presbyopia. You might also find that your night vision grows poorer. These changes usually occur simultaneously in both eyes.
Aging can also cause the lens to darken, grow opaque, and in some cases thicken, causing nearsightedness. Clouding of the lens, which is called cataract, usually develops slowly over many years. It may go unnoticed until the cloudiness blocks the central line of sight and impairs vision.
Over time, the anterior chamber (space behind the cornea and in front of the iris -- the colored part of your eye -- that is filled with aqueous humor) in each eye may become shallower in some susceptible individuals, raising the risk for blockage of the aqueous humor drainage system near the iris. This fluid backup may lead to increased pressure inside the eye that damages the optic nerve; this is called closed-angle glaucoma, which, if left untreated, can cause blindness.
The aging retina (the innermost layer of the eye) thins and may grow less sensitive because of cell loss, a reduced blood supply, or degeneration. Especially prone to deterioration is the macula (area of the retina responsible for sharp central vision); age-related macular degeneration is a serious disease that can steal a person's central vision.