How likely is it for an adolescent to have breast cancer?

Very, very unlikely: The incidence of breast cancer increases with age, occurring very rarely in one's 20's, sporadically in the 30's, and then increasing with each decade of life. An adolescent with a breast lump still requires physician evaluation, but a benign tumor called a fibroadenoma is far more likely.

Unlikely: Most cancer doctors have seen breast cancer as early as someone in their twenties. It doesn't mean it never happens earlier it's just that most of us never see it. Therefore it is extremely rare.