It can have many causes including age, injury or nerve malfunction. It can also occur at birth.
Age is the most common cause of ptosis.
The muscles that elevate the eyelid stretch and become thinned, resulting in a loss of muscle tone and the inability to hold the upper lid in the proper position above the eye. Injury is another common cause of ptosis. Trauma to the eye, such as during an automobile accident, can damage the delicate structures around and in the eye. Sometimes ptosis can be noticed at birth. In these cases it is due to an abnormality in the development of the muscles that elevate the upper lid.