Acne is caused by blockage in the hair follicles (tiny holes) in the skin of the face. Attached to these follicles, below the surface of the skin, are the sebaceous glands, which produce an oil called sebum to prevent the skin from drying out. When too much sebum is produced, it mixes with dead skin and other matter in the hair follicles, causing them to become clogged. This creates spots which can then become infected.

The reason why teenagers are more prone to acne than adults is that overproduction of certain hormones can lead to too much sebum, which in turn causes the problems described above. In adolescence mainly boys, but also girls, produce more testosterone than at other times, and this stimulates sebum production. Adult women also sometimes get acne, which again is often due to different hormones produced during periods or pregnancy.

You can learn more about the typical causes of acne here.