The ability of the eye to change its point of focus from far to near and vice versa is called accommodation. This involves activating a muscle (the ciliary body) and changing the shape of the lens inside the eye. The ability to use that muscle is very high in young children and steadily decreases with age, ultimately resulting in presbyopia. While children and young adults have a large capacity for accommodation, they do not necessarily have good control of the muscles. Sometimes they have a tendency to accommodate too much, sometimes too little, sometimes they cannot turn it on and off as quickly as needed (like when taking notes in class).
Accommodative disorders are treated with reading glasses and/or vision therapy. Often, children will outgrow the disorder; just because they need reading glasses one year does not mean they will have to use them forever.