Breast development begins in girls during puberty, usually between the ages of 8 and 14. The first sign of breast development is slight swelling under the nipple, a stage of development called breast buds. During this beginning stage, breasts begin to feel sore and tender due to the breast buds, breast growth, and development of mammary glands. By the end of puberty, most girls will have adult-like breasts and reached a more standard body weight. However, breast tissue may undergo further changes due to hormones, such as from pregnancy and menopause.

Breasts stop growing typically before the age of 18 when they reach their full size. However, some breast development may continue for some women into their early 20s. Everyone develops at a different rate, and fluctuations in weight can also play a part in breast size. It is not uncommon for one breast to be larger than the other after they stop growing, which is referred to as breast asymmetry and can be treated through corrective surgery if desired.

In conclusion, breasts usually stop growing by the age of 18, but some women may experience further growth into their early 20s. However, breast tissue may undergo further changes due to hormones, such as from pregnancy and menopause.