Your brain ages in five distinct stages, new research shows

Scientists identified the average ages—9, 32, 66 and 83—when the pattern of connections inside our brains shift. The brain’s adolescence phase, they discovered, lasts until age 32, and then it enters a period of stability until early aging begins at 66.

The study could help explain why brain-related conditions arise at certain ages, and could provide a better road map to understanding healthy aging.

“Maybe there is something going on that is changing within the brain structure or being optimized that sort of leaves us vulnerable for these specific things," said Duncan Astle, a professor of neuroinformatics at the University of Cambridge and co-author of the study, which was published in November in the journal Nature Communications.

The study’s authors examined results from about 4,000 brain scans taken from people in the U.S. and U.K., ranging from a newborn baby to a nonagenarian. The scans helped show white matter, the fatty substance that insulates the nerve fibers connecting brain regions. Researchers could see the physical connections and build a map of pathways that change over time.

They created an “average brain" for each year of life, Astle said. Then they examined a dozen characteristics and used a machine-learning algorithm to help pinpoint moments of significant change in the data.

The scientists determined the brain’s childhood period of development lasts from birth to age 9. During this time, it grows in size, but—because we are born with lots of excess wirings—our brains prune connections that aren’t used or as efficient, according to Tara Spires-Jones, director of the Center for Discovery Brain Sciences and professor at the University of Edinburgh, who wasn’t involved in the research.

At age 9, our brains enter the adolescent phase, which the study authors said lasts until age 32.

“Neuroscientists hate it whenever you say to them, ‘So, when is the brain mature? When is the brain adult?’" Astle said. “The really neat thing about this analysis is that it says when that is."

During this stage, the brain’s wiring becomes more efficient, and there is strong, rapid communication between regions as well as within them.

At 32, our brains enter the “adult phase," a period of consistency that lasts until 66. This time aligns with a plateau in our intelligence and personality, the researchers said, and the regions of the brain become more segregated.

Early aging begins after the next turning point. Between 66 and 83, some brain regions form stronger groups, known as modules, but they are less connected to other modules. White matter starts to degrade.

“Past age 65 or so there’s brain shrinkage and decreases in the integrity of white matter, and that corresponds to—for many of us, though not all—a slight decline in some aspects of cognitive function," Spires-Jones said.

Beyond 83, in the late-aging phase, the connections between regions deteriorate and our brains increasingly rely on individual regions with a small number of highly used interregional pathways, said Alexa Mousley, a University of Cambridge neuroscientist and lead author of the research.

She is intrigued by whether the brain is more susceptible to certain mental-health or neurological issues because of this rewiring, since there is a pattern between the five stages and common conditions.

Most autism diagnoses, for instance, are made in young children, according to the Centers for Disease Control and Prevention. Up to 75% of mental-health conditions begin by a person’s early 20s, per the Substance Abuse and Mental Health Services Administration. Alzheimer’s disease typically manifests during what the researchers call the early aging phase.

Cardiovascular health, social connectedness and exercise are all associated with positive cognitive health outcomes—and it’s possible they also play a role in the rewiring that happens later in life, too, Astle said.

There’s probably still a lot of individual variation around when these changes happen, said Katya Rubia, professor of cognitive neuroscience at King’s College London who wasn’t involved in the study. “This doesn’t mean we should all be starting to worry the moment we have our birthday and we are suddenly 83 years old," she says.

Write to Aylin Woodward at aylin.woodward@wsj.com