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

Not necessarily. When a girl's breasts first start to develop, they are said to be in the budding stage. This is usually between the ages of 10 and 14, but it can happen earlier or later. The budding stage is followed by the breast development stage, during which the breasts will continue to grow in size and shape. After a few years, the breasts will usually reach full maturity, although they may continue to change slightly over time.