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ages 12-18 Brain Systems During adolescence, an enormous pruning back of the millions of dendrites occurs, which speeds up and reorganizes the brain functions. At the same time, the brain initiates sexual maturation of the body, creating a maelstrom of fluctuating hormonal levels. Which teen behaviors are caused by brain development and which are caused by hormones? Scientists have been studying this question for years, and their findings have implications for anyone who knows a teenager.
It is suspected that “the excess of synapses means the young adolescent mind can't easily keep track of multiple thoughts, and it can't gain instant access to critical memories and emotions that allow grown-ups to make judicious decisions” (4). However, even without adult judgment, the dendritic pruning taking place throughout adolescence results in a brain (and body) that has faster reaction times, better memory, and increased speed in learning, as compared to adults (13). In other words, when teens think they’re smarter than their parents, it’s because they ARE, at least in some ways.
 diagram from The Brain website
The emotional, judgment, and cognitive systems in an adolescent brain are all maturing at different rates. While the cognitive system of a 16-year-old is at its absolute peak of speed and accuracy, and the limbic (emotional) system is highly activated, that 16-year-old’s Prefrontal Cortex (PFC) is still quite immature. “The prefrontal cortex is the seat of civilization" (4). It is able to manipulate ambiguity, moderate emotional responses, and make decisions. The maturation disparity between the cortex and limbic systems is responsible for many of the “How could you be so smart and do something that dumb?!” moments that most teens suffer through, even as the teen themselves think that they are way smarter than adults (16).
The interchange between the limbic system and Prefrontal cortex (PFC) is a ballet worth watching, because these two brain systems are of primary importance when dealing with teenagers. Neuroscientist Dr. Ronald Dahl (21) was quoted as saying the teenage brain is like “all drive and no brakes.” The “drive” of this equation comes from the limbic system. This is the emotional and willful area of the brain that is highly activated by hormones during the earliest years of childhood. During the ages of about 4 and 10, this system is relatively calm, but after approximately age 11, hormonal activity in the limbic system skyrockets up to adult levels. The limbic system is responsible for the emotional roller coasters and storms of adolescent will that most people associate with the teen years. The thinking done while under the influence of the limbic system is called “hot cognition”(21).
The “brakes” which are not yet entirely present in most teenagers is characterized by a fully-mature Prefrontal Cortex (PFC). The PFC doesn’t mature until about the early 20s, and later for many young men. The PFC acts as a mediator, lending good judgment to decisions, weighing consequences. When the limbic system decides to race bikes blindfolded down a steep hill, it’s the PFC that’s supposed to step up and say “Wait a minute…” The logical, reflective, wise decision-making process is called “cold cognition”(21). Unfortunately for many teens, the matured PFC won’t be present until about age 21 to impart judgment to decisions about sexual activity, drug use, and other harmful or just plain stupid behaviors. The relative intelligence of a teen is often not even a factor in the level of judgment they are able to demonstrate. By age 11 the hormonal affects of puberty are generally in full swing, while the maturation of the PFC won’t happen for another ten years.
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