Eyelid ptosis/drooping lid--what is usual cause?

Eyelid Drooping. This can be caused a defect present at birth or by gradual stretching of the muscles and connective tissue that hold open the eye. Surgery can usually correct the problem but it needs to be customized to your particular cause.
Droopy eyelid. There are several causes of a droopy eyelid. Your best bet would be to have your eyes evaluated by a board certified facial plastic surgeon.
Gravity. In older people, its just many years of gravity. In younger, it can be trauma or serious syndromes like myasthenia, horner's or third nerve palsy- one must see an ophthalmologist promptly.