When your breasts develop does that mean they have fully grown?

No. When your breasts develop it means that your breasts have begun to grow. The term developed breasts means that your breasts are fully grown and your breasts will be fully grown between the ages of 18 and 20.

However, size and shape may change after full development for reasons unrelated to puberty. For examples, weight change, pregnancy, gravity, and certain illnesses can affect breast size.