According to experts high blood pressure in the "critical period" of the 30s and 40s could "accelerate damage" to the brain.
A "window of opportunity" to safeguard brain health is to control high blood pressure in mid-30s to early-50s. a study says.
Following 500 people born in 1946, it linked higher blood pressure in early mid-life to later blood vessel damage and brain shrinkage.
This is not the first time that dementia risk due to raised blood pressure was adressed but according to experts high blood pressure in the "critical period" of the 30s and 40s could "accelerate damage" to the brain.
Throughout this study, published in Lancet Neurology, participants had their blood pressure measured and underwent brain scans.
Increases in blood pressure between the ages of 36 and 43 were associated with brain shrinkage.
Everyone's brain shrinks a little as they age, but it is more pronounced in those with neurodegenerative diseases like vascular dementia.
And while those studied did not show signs of cognitive impairment, the researchers say brain shrinkage usually precedes that - so they will be monitoring the people in the study over the coming years to watch for signs, reports BBC.
Raised blood pressure between 43 and 53 was also linked to more signs of blood vessel damage or "mini strokes" when in people reaching their 70s.
Prof Jonathan Schott, a clinical neurologist at the UCL Queen Square Institute of Neurology, led the research.
He said: "Blood pressure, even in our 30s, could have a knock-on effect on brain health four decades later. Monitoring and interventions aimed at maximising brain health later in life need to be targeted at least by early mid-life."
Prof Schott told the BBC: "NHS health checks are currently offered from the age of 40, and the uptake is, at most, 50%. Our data suggests blood pressure should be measured much earlier."
Paul Leeson, professor of cardiovascular medicine at the University of Oxford, said: "We have known for some time that people who have higher blood pressure tend to have different brain structure in later life.
"What doctors have been debating is whether treating high blood pressure in young people actually prevents these brain changes.
"The alternative, which is what we tend to do right now, is wait until later in life to start to take high blood pressure seriously because we know that by then, the more severe brain changes are definitely developing.
"These findings do support the idea that there may be critical periods in life, such as in your 30s and 40s, when periods of high blood pressure are accelerating damage within the brain."
Dr Carol Routledge, director of research at Alzheimer's Research UK, said: "High blood pressure in midlife is one of the strongest lifestyle risk factors for dementia, and one that is in our control to easily monitor and manage.
"Research is already suggesting that more aggressive treatment of high blood pressure in recent years could be improving the brain health of today's older generations.
"We must continue to build on this insight by detecting and managing high blood pressure even for those in early midlife."