What is meant by the least distance of distinct vision? How does this vary between the very young and old people?

(N/A) The least distance of distinct vision is defined as the minimum distance at which an object can be placed so that the eye lens can form a clear and sharp image on the retina without any strain. For a normal adult eye,this distance is approximately $25 \ cm$.
$(b)$ This distance varies with age because the flexibility of the ciliary muscles and the eye lens decreases over time. In very young people,the eye lens is highly flexible,allowing for a shorter least distance of distinct vision. In contrast,as people age,the lens becomes less flexible and the ciliary muscles weaken (a condition often associated with presbyopia),which increases the least distance of distinct vision.