Thelarche is the formation of breast tissue behind the areola (usually occurring in puberty), leading to the breasts that humans like so well. In a normal girl, thelarche begins with breast buds at anywhere between the ages of 8 to 14. Premature thelarche occurs when breast buds form before the age of 8 and can be a sign of precocious puberty or of an endocrine disorder.
See also menarche and adrenarche