The ciliary muscles, which help the eye lens focus, weaken with age, contributing to the development of presbyopia, commonly known as long-sightedness. 

Presbyopia: This is the age-related loss of the eye's ability to focus on nearby objects. 

Ciliary Muscles and Lens: The ciliary muscles control the shape of the lens, which is responsible for focusing light onto the retina. 

In an old man, long-sightedness (presbyopia) is primarily caused by age-related changes in the lens and ciliary muscles, not the retina. As we age, the lens loses its elasticity and the ciliary muscles weaken, making it harder for the eye to focus on close objects. While the retina can be affected by age-related diseases like diabetic retinopathy or macular degeneration, these are distinct from the age-related loss of accommodation (presbyopia).