**Core Concept:** The cornea is the transparent, outer layer of the eye that helps focus light onto the retina, protecting the eye from infections and injuries. Adult corneal size is crucial for its optical function and maintains its transparency.

**Why the Correct Answer is Right:** The cornea reaches its adult size by the completion of its growth, which typically happens during late adolescence or early adulthood. In humans, this occurs around the age of 18-20 years. The cornea grows in response to mechanical stress, such as the intraocular pressure, until it reaches its mature size.

**Why Each Wrong Option is Incorrect:**
A. This option is incorrect because the cornea continues to grow until late childhood or early adolescence. This is the period when the cornea is still being shaped by the intraocular pressure and other factors.
B. This option is also incorrect because the cornea starts to reach its adult size during puberty, but it continues to grow for several years post-puberty. The process of reaching full size is not complete by the age of 40.
C. This option is not correct because the cornea reaches its adult size during late adolescence or early adulthood, not in the mid-40s. The cornea continues to grow and remodel throughout life due to the effects of intraocular pressure and other factors.
D. This option is the correct answer because the cornea reaches its adult size during late adolescence or early adulthood, around the age of 18-20 years. After reaching its adult size, the cornea undergoes a process called keratocyte apoptosis, which helps maintain its transparency.

**Clinical Pearl:** Understanding the age at which the cornea attains its adult size is essential for diagnosing and managing corneal diseases or injuries affecting the transparency of the cornea in adults. For instance, if the cornea is opaque in an adult, it could be indicative of a disease like keratoconus or a traumatic injury, and prompt evaluation and treatment are crucial.

**Correct Answer:** D. Around the age of 18-20 years.