The researchers also suggest the changing pattern of gene expression could be linked to the development of schizophrenia in humans.
'The discovery of this genetic programme opens up a completely new way to understand behaviour and brain diseases throughout life,' said Professor Seth Grant at the University of Edinburgh, UK.
Researchers examined how genes in the brain are turned on and off across the lifespan of mice and humans. They analysed existing data on RNA taken from brain tissue samples of different ages, to detect when regulatory changes in gene expression occurred.
'We found that actual age could be predicted by examination of an RNA sample from mouse and human brain tissue,' the team reported in the journal eLife.
Their findings indicated a regulatory genetic calendar which controls how and when different genes are expressed. In humans, they were able to map changes from development in the womb to 78 years old.
The peak of gene expression reorganisation occurred around 26 years of age in humans, and the genes affected included those associated with schizophrenia. The researchers suggest this may be why people with schizophrenia do not show symptoms until young adulthood, despite having the genetic changes linked to the condition from birth.
Most changes in gene expression were completed by middle age. Women show a slightly delayed calendar of changes compared with men. The team found the genetic calendar is also present in mice, with more rapid changes over a shorter lifespan, suggesting the calendar is shared between species of mammals.
'Many people believe our brain simply wears out as we age. But our study suggests that brain ageing is strictly controlled by our genes,' said co-author Dr Nathan Skene, also at Edinburgh.
The team suggests its research could eventually lead to therapeutic drugs that modify the genetic lifespan calendar for young adult patients with psychiatric disorders.