Girls' bodies undergo gradual changes during puberty, analogous to but distinct from those experienced by boys. Puberty is the process of physical changes by which a child's body matures into an adult body capable of sexual reproduction to enable fertilisation . It is initiated by hormonal signals from the brain to the gonads -either the ovaries or the testes . In response to the signals, the gonads produce hormones that stimulate libido and the growth, function, and transformation of the brain, bones , muscle , blood , skin , hair , breasts, and sexual organs. Physical growth —height and weight—accelerates in the first half of puberty and is completed when the child has developed an adult body. Until the maturation of their reproductive capabilities, the pre-pubertal, physical differences between boys and girls are the genitalia , the penis and the vagina. Puberty is a process that usually takes place between the ages 10–16, but these ages differ from girl to girl. The major landmark of girls' puberty is menarche, the onset of menstruation, which occurs on average between ages 12–13.    
Testosterone is a steroid hormone produced in the adrenal glands of both sexes and in the testes of males and the ovaries of females. Testosterone is largely responsible for the formation and maintenance of male sex characteristics, including both the larger bone and muscle development seen in males. The testosterone levels in humans are regulated by hormones released from the brain; in males the hypothalamus and pituitary glands in the brain increase testosterone during puberty and male characteristics develop (for example, penile enlargement, facial hair, interest in sex).
AndroGel ® (testosterone gel) 1% and % are controlled substances, available by prescription, used to treat adult males who have low or no testosterone due to certain medical conditions. It is not known if AndroGel is safe or effective to treat men who have low testosterone due to aging . It is not known if AndroGel is safe or effective in children younger than 18 years old . Testosterone products that are applied to the skin may have different doses, strengths, or application instructions that may lead to different levels of testosterone in your body .