The defect of the eyes described in the question is called presbyopia. As we age, the ciliary muscles, which control the shape of the lens in our eyes, gradually become weaker. This weakens the ability of the lens to change shape and adjust its focus for near vision.

Presbyopia is different from myopia, astigmatism, and hypermetropia, which are other common refractive errors that affect the clarity of vision. Myopia, also known as nearsightedness, occurs when light focuses in front of the retina instead of directly on it, resulting in distant objects appearing blurry. Astigmatism occurs when the cornea or lens has an irregular shape, causing blurry or distorted vision at various distances. Hypermetropia, also known as farsightedness, occurs when light focuses behind the retina instead of directly on it, causing nearby objects to appear blurry.

In summary, presbyopia is the correct answer because it specifically refers to the age-related loss of flexibility in the lens, causing difficulty in focusing on close objects.