There are a number of ways that people change, quite significantly, between adolescence and adulthood. This includes physical changes to the body, as well as many changes in the brain that contribute to greater perspective and more realistic worldviews, as well as views of themselves and others. Each individual person may develop at a slightly different pace and to varying degrees, but there are a number of common factors that may be seen in the transition between adolescence and adulthood. Some of the most basic physical changes include increased height, improved strength, and the development of the reproductive organs.
Researchers typically break childhood and adolescence down into a number of developmental stages; some research even suggests that the brains are not fully developed until people are at least 25 years of age. This implies a much longer "adolescence" than was previously thought, though it clearly differs from the younger adolescence of the early and middle teen years. In most cases, the transition between adolescence and adulthood is defined as the period of time from the middle to late teen years, to the early to mid twenties. This is when people change the most, and will do the most maturing.
Physical changes should be expected between adolescence and adulthood. Most people will grow a bit in late adolescence, and will be finished growing by the time they are in their early twenties. The reproductive organs will develop completely, and adolescents will complete puberty, such as a changing or deepening of the voice, and the growing of facial and pubic hair. Hormone levels will also change between adolescence and adulthood, though this varies more uniquely between individual people; in general, however, the production of sex hormones will increase through late adolescence and into young adulthood.
Changes in the brain are also significant between adolescence and adulthood. Certain areas of the brain, such as the prefrontal cortex, are not fully developed in adolescents. As a result, their perception of risk is quite low, leading them to engage in riskier behavior than adults would; in addition, they often perceive themselves as more knowledgeable and more important than they actually are. As individuals transition from adolescence to adulthood, they will be able to more accurately determine the risks inherent in a situation, and adjust behavior accordingly. They will also gain additional perspective and often stop viewing themselves as the centers of their own worlds, but rather as participants in larger family units and community groups.