Breasts start to grow in most girls between the ages of 10 and 14. But there is huge variation and some girls' breasts may start to grow before this and some may start later - but the beginning of breast growth is often the first sign of puberty. Breast growth stops later than most people think. Research shows that your breasts can keep growing even into your 20s or 30s, though the major part of breast growth does occur during puberty - somewhere between 10 years and 18 years of age. Because breasts are made up in part of fat, if you gain or lose weight, your breasts can grow bigger or smaller. Each of your breasts will be a slightly different size than the other, just as one of your hands or one of your feet may be slightly bigger or smaller than the other. Also the position of one breast, and the way it is shaped may be slightly different.