You need to carefully unpick the lines in the first two stanzas to answer this question. Youth is depicted as a time of physical beauty. As one grows older, the cares and troubles of life are reflected in the worry lines that gradually appear on the face. Hair gradually loses its natural pigmentation, turning grey. Old people tire more easily, and doze during the daytime. Their physical actions are slower, as is their ability to absorb information - hence the need to read slowly.
(Sometimes younger people need to slowly read, too, in order to pick up subtle inferences and allusions - which is often referred to as reading between the lines).