Retinitis Pigmentosa (D): This is a group of genetic disorders that affect the retina's ability to respond to light, leading to a gradual loss of vision over years. The age of onset and the gradual progression of visual loss are consistent with retinitis pigmentosa.

Macular Degeneration (C): Although more common in older adults, some forms of macular degeneration can occur in younger individuals. It leads to a loss of central vision, which could explain the gradual decrease in visual acuity.
Glaucoma (A): Glaucoma can cause gradual vision loss if left untreated. It's more common in older adults but can occur at any age, especially if there's a family history.
Cataract (B): While cataracts are more common in older populations, they can occur in younger individuals due to various factors like trauma, certain medications, or systemic diseases. Cataracts can cause a gradual decrease in visual acuity.

Keratitis (E): Although less likely to cause a gradual decrease in visual acuity over years without other symptoms, chronic keratitis or corneal diseases could potentially lead to significant vision loss if not properly treated. Missing this diagnosis could lead to unnecessary vision loss.

Other Genetic or Inherited Retinal Disorders: There are numerous rare genetic disorders that can affect the retina and lead to vision loss. These would be considered if the more common causes are ruled out.
Choroidal Neovascularization: This condition can cause vision loss due to abnormal blood vessel growth in the choroid layer of the eye. It's less common in younger individuals without other risk factors like myopia or certain genetic conditions.
Inflammatory or Infectious Causes: Certain infections or inflammatory conditions affecting the eye can lead to gradual vision loss. These would typically be accompanied by other symptoms such as pain, redness, or sensitivity to light.