By the time you reach puberty, your brain is almost fully developed. Looking at a gymnastics troupe, we witness the changes inherent in puberty and how the body changes. Each girl takes their own unique path to adulthood. What determines when you start on this journey? Puberty occurs when a part of the brain called the hypothalamus begins to produce a hormone (gonadotropin) that has an effect on the testes and ovaries causing an increase sex hormone - estrogen in girls and testosterone in boys.