There are a couple of different causes of myopia, including the eyeball being too long or the cornea being too curved. According to WebMD, if either of these structural issues occurs, “the light that enters your eye won’t focus correctly. Images focus in front of the retina, the light-sensitive part of your eye, instead of directly on the retina,” causing blurry vision.
A person is more likely to develop myopia if one or both of their parents is nearsighted, as it is considered to be hereditary disorder. Nearsightedness commonly develops during childhood, but in some cases may not present itself until adulthood.