Final answer:
Shelley is experiencing puberty, which is the process of becoming sexually mature, involving the development of primary and secondary sexual characteristics.
Step-by-step explanation:
Shelley, age 12, is undergoing rapid physiological growth, and her reproductive functions and sex organs are maturing. This is referred to as puberty. During puberty, both primary and secondary sexual characteristics develop.
Primary sexual characteristics are those directly involved in reproduction, such as the testes in males and ovaries in females, while secondary sexual characteristics include traits like breast development in females and facial hair in males.
Hormones such as testosterone in males and estrogen in females play a significant role during this period. In females, menarche, which is the onset of menstrual periods, also occurs, typically around the age of 12 to 13.