Under normal circumstances, myopia does not usually occur after the age of 20. If myopia still occurs after this age, there are generally two reasons. The first one is unreasonable eye habits, such as always leaving the eyes in a state of excessive fatigue, using the eyes for a long time at close range, using computers and cell phones and other electronic products for too long, and often staying up late. These bad eye habits can change the refractive state of the eyes, leading to the appearance of myopia. The second type, which usually occurs in middle-aged and elderly people, is mainly due to the change in the refractive state of the lens during the development of cataracts, thus producing myopia of crystalline origin. These two conditions are the most common causes of myopia in adults.