Men's brains are nearly four years "older" than those of females the same age, explaining why women stay sharper for longer in old age, according to new research.
A study basing age on metabolism rather than birth date found an average 3.8 year difference between the two.
All brains get smaller with age, and it was already known that men's tend to shrink at a faster rate.
However, new research focused on metabolic processes that have a major influence on brain performance and ageing.
The US team looked at PET (Positron Emission Tomography) brain scan images from 205 men and women ranging in age from 20 to 82.
PET scans open a window to brain metabolism by measuring the flow of oxygen and glucose. The brain consumes large amounts of glucose sugar for energy, but the pattern of use alters with age.
For the study, the scientists used a machine learning computer programme to explore the relationship between metabolic and chronological brain age.
They found that metabolic brain ageing tracked chronological ageing in both men and women.
But analysis of the data showed that at any given age women's brains were younger, metabolically speaking, than men's — by an average of 3.8 years. The relative youthfulness of women's brains is detectable even among the youngest participants, said the researchers writing in the journal Proceedings of the National Academy of Sciences.
Lead scientist Dr Manu Goyal, from Washington University School of Medicine in St Louis, said: "It's not that men's brains age faster — they start adulthood about three years older than women, and that persists throughout life. What we don't know is what it means. I think this could mean that the reason women don't experience as much cognitive decline in later years is because their brains are effectively younger, and we're currently working on a study to confirm that."
The initial study was conducted by first training the machine learning algorithm using men's ages and brain metabolism data.
The programme was then directed to calculate each woman's brain age from its metabolism.
When the study was reversed — by training the algorithm on women's data and applying it to men — it produced a slightly different result. However, it still found men's brains to be 2.4 years older compared with women's.
In their paper, the researchers wrote: "We find that throughout the adult lifespan the female brain has a persistently lower metabolic brain age — relative to their chronological age — compared with the male brain.
"The mean metabolic brain age (minus actual age) was on average 3.8 years less for females compared with males."