Brain function is far from static throughout a person's life. From newborns to nonagenarians, human learning ability and the risk of cognitive decline are constantly changing. Now, scientists may have discovered a potential cause for this phenomenon: at four key age points—9, 32, 66, and 83—brain neural connections may undergo significant shifts. The relevant research findings were published in *Nature Communications* on November 25.
Previous research has shown that the human body experiences three rapid aging phases around the ages of 40, 60, and 80, but the complexity of the brain makes it even more difficult to understand.
The brain is composed of different regions that exchange information through white matter tracts, which are filamentous structures made up of axons—the thin processes extending from neurons. These connections influence cognitive functions, such as memory. However, it is currently unclear whether these neural connections undergo significant changes throughout life. Alexa Mousley of the University of Cambridge in the UK stated, "No one has previously combined multiple indicators to describe the phased changes in neural connections in the brain."
To fill this knowledge gap, Mousley's team analyzed MRI brain scans from approximately 3,800 individuals in the UK and the US. The participants were predominantly white and ranged in age from 0 to 90. The scan data came from various brain imaging projects, most of which excluded patients with neurodegenerative diseases or mental illnesses.
Research has found that brain connectivity in 90-year-olds typically goes through five main stages, separated by four key inflection points.
In the first stage, from birth to age 9, the white matter bundles between different areas of the brain appear to become longer or more coiled, resulting in reduced transmission efficiency. "Information takes longer to transfer between different areas," Mousley said.
This may be because the brain is filled with a large number of neural connections during infancy, but as children grow, unused connections are gradually "pruned" away. Mousley's analysis suggests that the brain seems to prioritize building extensive connections to accommodate skills such as playing the piano, but at the cost of reduced connection efficiency.
However, in the second stage, from age 9 to 32, this pattern appears to reverse, likely due to the onset of puberty and the effects of hormonal changes on brain development. "At this time, the brain suddenly increases the efficiency of neural connections; they become shorter, and information is transmitted faster," Mousley notes. This may contribute to the development of skills such as planning and decision-making, as well as improvements in cognitive abilities like working memory.
The third stage lasts the longest, from age 32 to 66. "The brain is still changing in this stage, but to a much smaller degree," Mousley explains. At this time, the efficiency of connections between different brain regions re-enters a period of gradual decline. "The exact drivers of this shift are not yet clear, but people in their 30s often experience many significant changes, such as having children and settling into a stable life, which could all play a role," adds Katya Rubia of King's College London. It could also simply be due to the body's natural wear and tear.
Researchers have found that in the fourth stage, between the ages of 66 and 83, connections between neurons in the same brain region appear to be more stable than connections between different brain regions. "This is interesting because the risk of developing dementia and other health problems gradually increases during this stage," Mousley said.
In the fifth stage, between 83 and 90 years old, connections between brain regions gradually weaken, and information is transmitted more through "hubs" connecting multiple areas. "This indicates that resources for maintaining connections decrease in this stage, and the brain relies more on specific areas as hubs for connections," Mousley said.
Mousley points out that understanding these brain changes helps explain why mental illnesses typically develop before the age of 25, and why people over 65 are particularly susceptible to dementia.
Rubia said, "Understanding the normal turning points in brain structure throughout a person's life is crucial so that we can explore abnormal changes that occur in mental health or neurodegenerative diseases. Once the abnormality is identified, we can pinpoint the treatment options. For example, we can investigate which environmental factors or chemicals cause the abnormality and look for ways to reverse it through treatments, policies, or drugs."
However, Rubia emphasized that further research is needed to verify whether these findings apply to more diverse populations.
Related paper information: https://doi.org/10.1038/s41467-025-65974-8
