If the plug enlarges and pops out, the tip looks dark and it's called a blackhead. This isn't dirt and it won't wash away. The darkness is due to a buildup of melanin, the dark pigment in the skin. If the process continues, a pimple forms. Acne most often starts at around age 11 for girls and 13 for boys.
Scientists think a hormone called androgen plays a role in acne.
Among other things, androgen stimulates the sebum-producing glands. After puberty, boys produce 10 times as much androgen as girls, and so it's not surprising that more boys than girls develop severe cases of acne. Also, bacteria called Corynebacterium acnes, which cause skin fats to break down into irritating chemicals, can directly contribute to an outbreak. Some oily cosmetics and shampoos can, on rare occasions, trigger acne in people who are prone to get it. The American Academy of Dermatology says it's a good idea for acne sufferers to check with a dermatologist to ensure the skin condition really is acne.