Do human visual functions degenerate due to genetic factors or by external factors?
Vision deteriorates for both reasons, but I'm not quite sure how to separate the effects of aging from wear and tear. 

Cataracts are the leading cause of blindness in the world. Ways in which environment (which falls under the category of wear and tear in my book) affects cataract formation: 


UV light: people living at high altitudes (e.g. Tibet) have more cataracts 2° to increased UV light than those at lower altitudes (sunglasses with UV protection delays the process)
lead exposure increases cataract formation.
infections in infancy and childhood (rubella, toxoplasmosis, other)


Macular degeneration is another cause of vision loss. Environmental factors include:  


smoking: speeds up age related macular degeneration.
diet: diets low in antioxidants, and obesity increase degeneration.


Retinal damage caused by diabetic retinopathy:


Juvenile onset diabetes has long been thought to be associated with viral infections from coxsackievirus B, cytomegalovirus, adenovirus, rubella, and mumps.
Type II diabetes is influences by obesity, lack of exercise, certain medications, (nicotinic acid, some diuretics, anti-epileptics, psychiatric drugs, and drugs to treat HIV (can impair beta cells or disrupt insulin action); Pentamidine (used to treat a type of pneumonia) can cause pancreatitis and diabetes; glucocorticoids/steroid hormones may impair insulin action. 


Corneal opacities:


infections: (Chlamydia trachomatis) bacteria that are spread by close contact with an infected person or by flies, especially common in Africa.  
recurrent injury (soot and pathogens from indoor smoke, a dusty, windy environment).  
vitamin A deficiency


Nearsightedness:
- near-point work: reading, working with a computer all day, etc.

Glaucoma:
- diabetes, obesity, near-point work, other.

Nonenvironmental causes would be genetics and illnesses associated with aging.


genetic predispositions associated with diabetes, nearsightedness and farsightedness
decreased lens plasticity with age
strokes secondary to hypertension, etc.
amblyopia (“lazy eye”), strabismus (when eyes are aligned in different directions), astigmatisms 
hereditary causes of glaucoma and age-related macular degeneration
retinitis pigmentosa, (degenerative disease of the retina)
innumerable genetic disorders (Marfan's, Tay-Sachs, Albinism, Cone-rod dystrophy, various autoimune disorders, etc. 
retinoblastoma
predisposition to glaucoma


 Focusing on Vision Through an Environmental Lens
 Glossary of Eye Conditions
 Causes of Diabetes
 Inherited Eye Disease