Throughout life, the brain is continually changing and "rewiring" itself. A bulk of the rewiring occurs from about the age of 12 up until about age 25.
Have you ever heard of the "Teenage Brain?" That is, the inane ability of teenagers to make the worst decisions despite knowing right from wrong, to push boundaries, to act impulsively, to take incredible risks? Although quite frustrating as a parent, you will be happy to know that this is all quite normal behavior for teenagers.
To understand why teenagers behave that way, let's back up a little. About 20 years ago, (rather recently in the science world), NIH (National Institutes of Health) scientists determined that the brain undergoes "massive reorganization" around ages 12 and 25. Imagine that for 12 years, your house was wired electrically in one particular order. Then, 12 years later, some wiring changes, and another 12 or so years after that, other wires change.
As a parent, this helps explain A LOT!
The brain might be almost the size of an adult's by the age of 6, but the connections, or wires, of the brain (neurons) are being optimally organized up until age 25. This rewiring process takes place from back to front (see figure on the side). As a baby and toddler, the brain is working hard to develop gross motor skills (sitting up, crawling, walking, eating) and verbal skills (which is why learning a foreign language is easier for little children).
During adolescence (until age 25), the focus of development progresses to the middle part of the brain, the limbic system (dopamine reward system) and then slowly to the front part of the brain, the prefrontal cortex (judgment, reasoning, impulse control). I often refer to these 2 areas being similar to a the engine (dopamine reward system) and brakes (prefrontal cortex) of a race car. During adolescence, again that is up until around age 25, the engine (the dopamine reward system) is highly charged, highly revved up and becoming finely tuned while the brakes (the prefrontal cortex) are practically non-existent at puberty but are being built slowly over time and are not fully working until age 25.
1. The dopamine - reward system. Connections in this part of the brain are being vigorously fine-tuned and are responsible for pleasure or reward. This part of the brain exists for "survival of the species." For example, dopamine is released when you eat, drink, sleep, have sex, bond with your child, etc. so that you feel good and want to keep doing them - all in an effort for the humans to survive. Essentially, when a behavior causes dopamine to be released in the brain, you will want to repeat that behavior to feel good again. So, when a child does well on an exam, scores the winning goal, or is commended by you for cleaning up the garage, the neurons in this part of the brain are further strengthened such that your child will want to repeat that behavior because the reward (the dopamine release) felt really good. This is also why "positive parenting" or choosing to point out and focus on something positive your child did rather than yelling or screaming when your child makes a poor choice can be an effective parenting technique.
2. The Prefrontal Cortex The brakes. Why is that when our children know that it is safer to wear a helmet or knows that texting something mean to a classmate will hurt their feelings that they choose to do the opposite? This can be attributed to the very slow wiring process of the part of the brain that controls judgment and impulse control - the prefrontal cortex. Remember, those brakes are faulty until about age 25!
Essentially, the brain is trying to "survive" by learning and thus reinforcing behaviors that make you feel good; yet, the ability to judge whether or not that behavior is actually GOOD for you (the job of the prefrontal cortex/brakes) is not quite fully functioning. So ANYTHING that causes dopamine release will be reinforced, and that includes the use of substances (nicotine, marijuana, alcohol, certain prescription medications, etc). As a parent, you must help guide your child to find behaviors that will reinforce that reward in positive ways so that he/she will not turn to alcohol, marijuana, nicotine, etc.
Here is are some links to sources if you would like to understand what is happening in your child's brain and behavior even further: