An age-related condition - when switching focus between distant and up-close objects becomes difficult.
As children, our ability to switch focus freely and with perfect clarity between distant and up-close objects was imperceptible. But as we age, that ability diminishes and the switch in focus becomes more noticeable and more difficult.
The reason for this comes down to our natural lens' loss of elasticity; resulting in the need to wear different glasses for different focal lengths – near and far. Those that can currently see well at a distance will require glasses to assist close-up focus and vice-versa. This is called Presbyopia, a condition that becomes common once we reach our late 40s.