For example, less than 3% of girls have not yet achieved thelarche by 13 years of age. Developmental changes during puberty in girls occur over a period of 3 – 5 years, usually between 10 and 15 years of age. They include the occurrence of secondary characteristics beginning with breast development, the adolescent growth spurt, the onset of menarche – which does not correspond to the end of puberty – and the acquisition of fertility, as well as profound psychological modifications.