Growing gray hair at 40 can be a normal phenomenon, but sometimes it can be caused by pathological reasons (such as malnutrition, vitiligo, genetic factors, etc.). In most cases, physiological reasons such as staying up late, high mental stress, excessive anxiety, etc. may cause 40-year-olds to grow gray hair, and even younger people may grow gray hair due to physiological reasons. In a few cases, some pathological reasons may also appear white hair, for example, malnutrition, vitiligo, hereditary factors, etc., which need to be treated under the guidance of a doctor for the original cause. If you have a lot of white hair, we recommend that you go to the hospital for a timely examination to find out the cause of the disease and then treat it.