Most people reach the end of their breast development at the end of puberty. The length of puberty may vary slightly, but generally speaking, the glandular part of the breast has completed its development by the age of 16-17. The breasts are not made up of glands alone, but also fat, and if you gain weight in the future, your breasts may increase in size accordingly. In addition to puberty, the glands are still affected by various hormones during pregnancy and breastfeeding, and some people may have the possibility of secondary growth.