Adolescence is a period of life when the body undergoes changes, leading to reproductive maturity. It is the transition period between childhood and adulthood, where physical and mental development occurs. Adolescence begins around the age of 11 and lasts up to 18 or 19 years of age. During this period, there are changes in height, weight, sex organs, brain structure and function, and muscle mass. The most important change that marks puberty is that boys and girls become capable of reproduction. In girls, adolescence may begin a year or two earlier than in boys. The human body undergoes several changes during adolescence, and these changes mark the onset of puberty.