The correct answer is reaching a critical weight level.
The advancement and onset through the different phases of puberty are affected by various factors. In both humans and animals, the age of puberty seems to be associated more with the weight of the body than with the chronological age.
Low body fat and undernutrition, or an altered ratio of body fat to lean mass, appears to delay the adolescent spurt and to slow the beginning of menarche.
The critical concentration of fat of the body suggests that a specific body composition, in supplementation with other psychosocial and environmental elements, is essential in sustaining and initiating the process of puberty.