Diseases/Medications. Adding to dr. May's answer - older folks also tend to have more conditions/diseases like thyroid disease, arthritis, etc and these can lead to ocular surface disease/dry eye. Medications, like blood pressure meds, diuretics, and antacids (like nexium) can cause dry eye. And the longer you live, the more you have been exposed to environmental effects of photochemical pollutants and pollen.
Dry eye. Hormonal changes, natural age related decrease in tear film/lid glands, medications, dietary issues, systemic disease, etc.