Human eyes reach nearly their full adult size by the time a child is two years old.

At birth, a human eye is about 17 millimeters wide. This is already 70% of the size it will be when the person is an adult. Most of the remaining growth happens during the first two years of life. By age three, the eye has reached about 90% of its final volume.The cornea is the clear front part of the eye. It reaches its full adult width of about 12 millimeters by age two. In comparison, the rest of the human body grows much more. An average person's height increases by about 3.5 times from birth to adulthood.The eye stays a similar size to keep vision clear. The lens must project light at a specific distance onto the back of the eye called the retina. If the eye grew as much as a leg or an arm, the focus would constantly change. This would make it very hard for the brain to process clear images.While the eyeball mostly stops growing early, the lens inside continues to change. It adds layers over time, similar to how an onion grows. The eye socket also expands slightly to make room for protective fat around the eyeball. Most changes in vision during teenage years come from the eye getting slightly longer, not wider.An adult eye is usually about 24 millimeters long and weighs about 7.5 grams. This stable size helps the optical system work correctly throughout a person's life. It ensures that the delicate parts of the eye remain in the right place to capture light.