Puberty: collection of physical changes that marks the onset of adolescence, such as growth of breasts or testes and the growth spurt. Timing differs by gender: girls, begin their growth spurt at about age 11, reach peak rate of growth at about 12, achieve mature stature at about 15, boys experience each of these about two years later. At age 11 and 12, girls are taller and more mature-looking: eventually, males are about 15 centimetres taller. Some aspects of brain development are nearly complete in adolescence: increase in size, myelination, synaptic pruning. The pleasure and reward systems in the limbic system reach maturity in early adolescence: areas of the frontal cortex associated with behavioural control are still developing. Primary sex characteristics: physical signs of maturity directly linked to the reproductive organs. Secondary sex characteristics: physical signs of maturity not directly linked to reproductive organs. Both of these occur, on average, at age 13.