What causes brown spots?

Drew explains in a YouTube video that age (or sun) spots are caused by ultraviolet rays from the sun. When your skin is exposed to ultraviolet rays, the body produces melanin as a way to tan the skin, effectively building up an armor to protect the deeper layers of skin. This is why age spots typically appear on the arms, neck, shoulders, face, and other areas of the body that are most often exposed to the sun.
While sun spots are not ordinarily dangerous, if you notice some are much darker than others, have been growing quickly, or have oddly shaped borders, you should visit a doctor. If they are red, tender, or bleeding, you should see a doctor immediately.