Children and teenagers act more impulsively than adults partially because of what?

The developmental stages of school-age children and teenagers explain their tendency to be more impulsive in their actions than adults.

The phrase social cognition generally refers to “the various psychological processes that enable individuals to take advantage of being part of a social group”
(Social cognition. Frith CD Philos Trans R Soc Lond B Biol Sci. 2008 Jun 12; 363(1499):2033-9, p. 2033)
This applies to both school-age children and adolescents.

1. Children ages 6 years to 12 years find acceptance among their
peers more valuable than anything else. To be welcomed or to belong, school-age children are willing to participate in specific behaviors. They may experiment on behaviors such as stealing, lying, or cheating. School-age children perform such impulsive behaviors as they learn and work out the rules set by society, friends, family, and school. At this stage of development, the child already has the confidence to face any trial. This feeling of individual power assures the child that he or she can be creative, get results for hard work, and solve issues without help.
2. Teenagers are susceptible to exhibiting risk behaviors because of peer pressure, weak self-control, and certain behaviors that seek various sensations. This boldness is usually a challenge for adults. Consequences for such actions are constant sources of financial, mental, and emotional stress. Teenagers also have the capacity to think more about how other people regard them. This mindset, together with their physical and emotional transitions, cause the teen to think that people think about what’s in their heads and not about who they are. In their minds, they have an audience that is real. They base their behavior to match what their so-called “imaginary spectators” expect of them.
School-age children and adolescent stages are the bridges on which children of six or five years cross to reach adulthood. Impulsiveness is part of their discovery as they become more of the person they are truly meant to be.

There are actually several theories around to attempt to explain the impulsive nature of many teens. This is one of those scientific research areas that is always evolving over time and is constantly being fine-tuned through further research. It has also been noted in scholarly journals that it is important to classify what exactly constitutes impulsivity, as that looks different from child to child based on interests and overall personalities. Here are some current ideas that scientists are still working on:
1. The immaturity of the prefrontal cortex. Up until about a decade ago, it was standard belief that teenagers (and younger children) acted impulsively because the prefrontal cortex region of their brains was underdeveloped (and would reach maturity around age 25). Scientists agree that this region of the brain affects decision making, social choices, personality, and some cognitive functions. Researchers looked at sophisticated images of teenagers' brains as they developed into young adults and noticed that there were significant changes in this area of the brain throughout that decade of life and determined that as this region changed in development, teenagers grew in their abilities to regulate their behaviors and curb their impulses. Eventually researchers noticed a discrepancy to this theory: teenagers who know the bad outcomes of their risky behaviors ahead of time are less likely to actually engage in potentially risky behaviors. (Younger children do not show this same tendency.) So it seems that there is more than an underdeveloped prefrontal cortex that creates impulsive teenagers.
2. Rising levels of dopamine in the brain. Dopamine is an important neurotransmitter that conveys information from nerve cells to the nervous system. It is also associated with rewards, compulsion, and pleasure (among other things). Because teenagers are experiencing a surge in dopamine levels as they grow, they have a tendency to reach out for experiences that are novel and exciting.
3. Teens simply lack the wisdom of life experiences. This isn't an especially complex idea, but there is validity to the claim that people learn from their mistakes. And as they learn from giving in to impulsive choices that lead to bad outcomes, they also learn to make different choices when similar situations arise. This doesn't mean that teenagers necessarily lack the biological structures to curb impulses; it simply implies that they haven't made enough poor choices along the way to know to choose differently.
The famous marshmallow test has been updated using money as the tangible reward. When participants are given the option of either receiving a smaller monetary sum immediately or a larger sum after a time period has passed, teens more often choose the immediate sum of money than do adults. Again, this likely varies depending on both the background of the teens and each teen's personality, which is why studies on impulsivity are so difficult to classify. What creates impulsive behaviors in one teenager or child will not create strong reactions in another. The complexities of the human brain are always being studied to assess the reasons behind various human behaviors.