What are the small flat grey skin patches all over my legs?

Keratoses: They are likely benign lesions known as stucko keratoses. These generally occur in people of your age group. Because they're a wide variety of skin lesions which can appear gray in color, it is a good idea to see your dermatologist. In the office, they may feel and see the lesions you are describing. Although rare, some potentially dangerous skin conditions can appear this way as well.