Age limits play an important role in various areas of law. In this chapter, we look at the psychological, biological and neuroscientific research on puberty, adolescence and adulthood. According to new proposals, adolescence extends from the age of ten to 24. While various studies suggest that cognitive development has reached adult level by the age of 16–20, researchers assume a longer process for emotional and social maturity. These views are compared with new findings on brain development. The question of the extent to which “typical” adolescent behavior is a reaction to psychosocial circumstances is also addressed. A specific “adolescent brain” cannot be identified based on current knowledge.