During puberty, both men and women produce more of the male hormone testosterone. This increases the production of sebum in your skin, causing it to become greasy and encouraging spots. It's thought that this is one of the main reasons most teenagers develop acne during puberty. But if you have acne, it doesn't always mean that you have high levels of testosterone. Instead, your body may be more sensitive to the effects of testosterone.
If you're a woman, hormonal changes just before your period may trigger acne. Newborn babies can sometimes get acne in the first few months of life.