Something almost everyone gets at one time or another. It's most common in adolescents who are going through puberty, and is due to the abundance of hormones flowing through their bodies. Usually it starts in early teens and ends in the early to late twenties, occasionally it will extend past thirty and sometimes even into someones forties or fifties.
Acne is not due to dirt, oily foods, or chocolate/sugar. Milk has been linked to increase in acne, however. Stress/anger can cause hormonal changes which can cause acne. Excessive touching of the face is also a bad idea. Genetics also play a key role.
Washing ones face will help, however people who have acne don't necessarily not wash their faces. If the case is especially serious, one should consult a doctor to be put on an anti-acne drug. Common ones include: Erythromycin, tetracycline, and clindamycin.
Bobby has had acne since he was 12. He's on a topical, and oral solution. He visits a dermatologist every two months. He still gets acne though, i'm glad i don't have his gene pool.