The five ‘ages’ of the human brain: Crucial changes occur around the ages of nine, 32, 66 and 83

The human brain is home to some 86 billion neurons. These are the “mysterious butterflies of the soul,” as Nobel laureate Santiago Ramón y Cajal called them, the principal cells of the nervous system, responsible for carrying all the information that allows us to think, laugh, remember, or breathe. These butterflies communicate, Cajal said, through “kisses,” the synapses, weaving sophisticated connections to transmit the nerve impulses that build life.

But this network of neural pathways that populate the brain is not static; it changes and reconfigures itself throughout life. Research published Tuesday in the journal Nature Communications has delved into how these structures are organized over time and has identified five ages of the human brain — that is, five distinct periods of neural development. The authors, a group of scientists from the University of Cambridge (UK), concluded that crucial changes occur in this arrangement of neural networks around the ages of nine, 32, 66, and 83.

After comparing the brains of more than 3,800 people aged nought to 90 using MRI scans that map neural connections, scientists identified four key turning points that mark the beginning and end of “brain ages.” This finding is significant, especially considering that the way the brain is wired is linked to neurological, mental, and neurodevelopmental disorders. “By understanding these key turning points, we can better understand what the brain is most vulnerable to at different ages. The more we learn about the expected changes in brain connections throughout life, the better we can distinguish what is considered a healthy and typical change from signs of something related to an illness or disorder,” explains Alexa Mousley, the study’s lead author.

The first turning point identified by researchers occurs around age nine. Until then, they maintain, children’s brains undergo a “consolidation of the neural network,” where the most active synapses survive and there is an increase in both gray matter (containing neurons) and white matter (composed of connections). But at the end of this first childhood phase — coinciding with the onset of puberty — the brain experiences a radical change in its cognitive capacity and in socio-emotional and behavioral development.

The second identified stage, which the authors call “adolescence,” spans from nine to 32 years of age. During this period, the organization of all the neural wiring remains more or less constant: this entire network becomes increasingly refined, and the connections become increasingly efficient.

However, the fact that this second phase lasts until the age of 30 does not mean that the brain is adolescent until then, clarifies Sandra Doval, a research professor at the International University of La Rioja. Speaking to the SMC Spain website, the researcher, who was not involved in this study, explains that “the study identifies when the patterns of brain wiring reorganization change, not when the brain matures, ages, or declines in functional terms.” In fact, the authors themselves point out that “the transition to adulthood is influenced by cultural, historical, and social factors,” making it a change more dependent on context than on biology.

The “strongest” change

At age 32, Cambridge researchers identify another turning point, the “strongest change in the organization of neural networks in life,” they say. This coincides with the peak in white matter maturation — other studies had already pointed out that brain connectivity peaks in the early 30s — and the changes in the architecture of the neural network, which until then had been occurring rapidly and begins to slow down. This age of the brain is the longest stage, ranging from 32 to 66 years old. “This period of network stability also corresponds to a plateau in intelligence and personality,” the authors agree.

There is another turning point at age 66, coinciding with a significant shift in health and cognition in high-income countries, scientists note. In fact, it is from this age onward that dementia or hypertension can begin to appear, which is also linked to cognitive decline and accelerated aging. This initial phase of aging lasts until age 83.

Around that age, the last of the inflection points identified by the Cambridge researchers occurs, marking the beginning of the brain’s final stage. Although they admit that data on this phase is limited, they do observe that different areas of the brain have more difficulty communicating.

Mousley asserts that the way the brain changes its connections throughout life could help “better understand the related changes in cognition and behavior.” “Understanding these fluctuations could help us understand how people change throughout their lives and why they are vulnerable to different disorders at different ages,” the researcher explained in an email response.

Rafael Romero García, director of the Neuroimaging and Brain Networks Laboratory at the University of Seville, told SMC that it is “a rigorous study” and, although it has limitations acknowledged by the authors — for example, the analysis was not separated by gender and men and women could have different developmental rates — he points out: “It is a great contribution that has allowed us to identify turning points in development and that could help us better understand the brain alterations associated with neurodevelopmental disorders and dementia.”

“Strict boundaries”

Romero García, who also did not participate in the study, clarifies, however, that these stages in brain maturation should not be interpreted as “strict boundaries.” “The distinction between maturation and aging is relatively arbitrary. Furthermore, it’s important to keep in mind that the study only focuses on brain connectivity; it doesn’t analyze how cognitive aspects such as learning, memory, problem-solving abilities, etc., change during these stages,” he points out.

According to Sandra Doval, “the results fit remarkably well with known milestones in neurodevelopment and aging,” but she also emphasizes the limitations of the research — another example: those over 60 are likely healthier than the average for their age, and that might not accurately represent typical aging — and urges caution in interpreting the findings, although she acknowledges their “scientific relevance.” “These findings do not generate immediate, direct clinical recommendations, but rather establish a valuable scientific context for future research on critical windows for preventive or therapeutic intervention at different life stages.”

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