The teenage years can be mystifying for parents. Sensible children turn scatter-brained or start having wild mood swings. Formerly level-headed adolescents ride in cars with dangerous drivers or take other foolish risks.

A flood of new research offers explanations for some of these mysteries. Brain imaging adds another kind of data that can help test hypotheses and corroborate teens’ own accounts of their behavior and emotions. Dozens of recent multiyear studies have traced adolescent development through time, rather than comparing sets of adolescents at a single point.

The new longitudinal research is changing scientists’ views on the role parents play in helping children navigate a volatile decade. Once seen as a time for parents to step back, adolescence is increasingly viewed as an opportunity to stay tuned in and emotionally connected. The research makes it possible to identify four important phases in the development of intellectual, social and emotional skills that most teens will experience at certain ages. Here is a guide to the latest findings:

Ages 11 to 12

As puberty takes center stage, tweens can actually slip backward in some basic skills. Spatial learning and certain kinds of reasoning may decline at this stage, studies show. Parts of the brain responsible for prospective memory, or remembering what you are supposed to do in the future, are still maturing. This may be why a teen may seem clueless if asked to give the teacher a note before school.

Coaching tweens in organizational skills can help. Parents can help build memory cues into daily routines, such as placing a gym bag by the front door, or helping set reminders on a cellphone. They can share helpful tools, such as task-manager apps.

Young teens’ reasoning and decision-making skills often aren’t fully developed; parents can coach them in being organized and considering other points of view. Illustration: Robert Neubecker

Parents can help foster sound decision-making, thinking through pros and cons and considering other viewpoints. Children who know by age 10 or 11 how to make sound decisions tend to exhibit less anxiety and sadness, get in fewer fights and have fewer problems with friends at ages 12 and 13, according to a 2014 study of 76 participants published in the Journal of Behavioral Decision Making.

By remaining warm and supportive, parents may be able to influence the way their teen’s brain develops at this stage. A 2014 study of 188 children compared the effect of mothers ​who were warm, affectionate and approving during disagreements, versus mothers who became angry and argumentative. Teens at age 16, who had affectionate moms when they were 12, ​showed brain changes linked to lower rates of sadness and anxiety and greater self-control, according to the study led by researchers at the University of Melbourne in Australia.

Ages 13 to 14

Parents should brace themselves for what is often a wildly emotional passage. Young teens become sensitive to peers’ opinions and react strongly to them. Yet the social skills they need to figure out what their peers really think won’t be fully mature for years, making this a confusing and potentially miserable time.

At about this time, teens’ response to stress goes haywire, sparking more door-slamming and tears. The impact of social stress is peaking around this time: Of adults with mental disorders often triggered by stress, 50% received a diagnosis before age 15. Other research shows teens from ages 11 to 15 become sad and anxious when subjected to social stresses such exclusion from social groups, while adults don’t show a similar effect.

Parts of the brain most vulnerable to stress are still maturing, so coping strategies teens use at this stage can become ingrained in the brain’s circuitry as lifelong patterns, according to a 2016 research review in Developmental Science Review. Psychologists advise teaching and modeling self-soothing skills, such as meditation, exercise or listening to music.

Teens are susceptible to social stress at ages 13-14. Parents can help decode peers’ social cues and model healthy coping behavior, like exercise or meditation. Illustration: Robert Neubecker

Coach teens on friendship skills, including how to read their peers’ expressions and body language. Encourage them to choose friends based on shared interests, not popularity, and to dump friends who are unkind. Teach them how to repair friendships after a fight by apologizing, making amends or compromising.

Family support is a stress buffer. Teens whose families provide companionship, problem-solving and emotional support are less likely to become depressed after exposure to severe stress, according to a 2016 study of 362 Israeli adolescents in the Journal of Family Psychology.

Ages 15 to 16

Teens’ appetite for risk-taking peaks at this age, according to a 2015 study of more than 200 participants ages 8 to 27 led by researchers at Leiden University in the Netherlands. The brain’s reward receptors are blossoming, amplifying adolescents’ response to dopamine, a neurotransmitter associated with feelings of pleasure and satisfaction. This makes thrill-seeking more desirable than it will ever be again.

Normal fears of danger are temporarily suppressed during adolescence, a shift scientists believe is rooted in an evolutionary need to leave home and explore new habitats. Studies have found adolescents fail to change their appraisal of risky situations even after being warned that the hazards are greater than they expect.

The ability to make and keep good friends is especially useful at this stage. Teens with friends they trust and count on for support are less likely to engage in risky behavior such as shoplifting, riding with a dangerous driver or having unprotected sex, according to a 2015 study of 46 teens led by Dr. Eva Telzer, an assistant professor of psychology at the University of Illinois in Champaign-Urbana. Teens who argue often with close friends are more likely to take such gambles.

Thrill-seeking will never be more irresistible than it is for a 15- to 16-year-old, whose reward receptors in the brain are blossoming. Parents can still make a difference: Encourage healthy friendships; show warmth and support. Illustration: Robert Neubecker

It is not too late for warm, supportive parents to make a difference. In a laboratory risk-taking test, teens who grew closer to their parents starting at age 15 showed less activation of a brain region linked to risk-taking and took fewer chances 18 months later, according to a 2015 study of 23 adolescents published in Developmental Cognitive Neuroscience. The closeness to parents included having parents’ respect and help talking through problems, and an absence of arguing or yelling, according to the study, in which Dr. Telzer was a co-author.

Ages 17 to 18

Benefits of the teenage brain’s ability to change and develop are evident at this stage. Some teens show increases in IQ. Intellectually gifted teens are most likely to achieve gains in IQ scores, so teens who are already smart are likely to grow even smarter, according to a 2013 study of 11,000 pairs of twins led by researchers at Penn State University, in University Park, and the University of Colorado at Boulder.

Older teens can put the brakes on emotions and risk-taking; their problem-solving and strategy-planning skills are developing. They might need help deciphering ambiguous people and situations. Illustration: Robert Neubecker

In older teens, the parts of the prefrontal cortex responsible for judgment and decision-making typically are developed enough to serve as a brake on runaway emotions and risk-taking. Executive-function skills, such as solving problems and planning strategies, continue to develop at least through age 20, according to a 2015 study by researchers at Sheffield Hallam University, England.

Social skills and related brain regions are still maturing, according to researchers including Sarah-Jayne Blakemore, a professor of cognitive neuroscience at University College London. At this stage, teens are better at noticing how others feel and showing empathy. They still lack the ability to decipher people’s motives and attitudes in complex social situations, though, such as figuring out why a friend might suddenly change the subject during a conversation at a party.

Write to Sue Shellenbarger at sue.shellenbarger@wsj.com