Mental powers start to dwindle at 27 after peaking at 22, marking the start of old age, US research suggests.
Professor Timothy Salthouse of the University of Virginia found reasoning, spatial visualisation and speed of thought all decline in our late 20s.
Therapies designed to stall or reverse the ageing process may need to start much earlier, he said.
His seven-year study of 2,000 healthy peopleaged 18-60 is published in the journal Neurobiology of Aging.
To test mental agility, the study participants had to solve puzzles, recall words and story details and spot patterns in letters and symbols.
The same tests are already used by doctors to spot signs of dementia.
In nine out of 12 tests the average age at which the top performance was achieved was 22.
The first age at which there was any marked decline was at 27 in tests of brain speed, reasoning and visual puzzle-solving ability.
Things like memory stayed intact until the age of 37, on average, while abilities based on accumulated knowledge, such as performance on tests of vocabulary or general information, increased until the age of 60.
Professor Salthouse said his findings suggested "some aspects of age-related cognitive decline begin in healthy, educated adults when they are in their 20s and 30s."
Rebecca Wood of the Alzheimer's Research Trust agreed, saying: "This research suggests that the natural decline of some of our mental abilities as we age starts much earlier than some of us might expect – in our 20s and 30s.
"Understanding more about how healthy brains decline could help us understand what goes wrong in serious diseases like Alzheimer's.
"Alzheimer's is not a natural part of getting old; it is a physical disease that kills brain cells, affecting tens of thousands of under 65s too.
"Much more research is urgently needed if we are to offer hope to the 700,000 people in the UK who live with dementia, a currently incurable condition."