Some factors are more of a risk than others, the report suggests.
For example, it estimates that hearing loss and high cholesterol are responsible for most cases of preventable dementia (7% each).
In early life it is lack of education that tips the balance while, in later life, social isolation and failing eyesight are a big risk.
Some experts are more cautious about what the evidence shows.
Prof Tara Spires-Jones, director of the Centre for Discovery Brain Sciences at the University of Edinburgh, said this type of research could not conclusively link any of these factors directly to dementia.
However, she said it would contribute to growing evidence that a healthy lifestyle “can boost brain resilience and prevent dementia”.
“We should be careful not to imply that people with dementia could have avoided it if they’d made different lifestyle choices,” said Prof Charles Marshall, from Queen Mary University London. He added that most of an individual’s risk of developing dementia is outside their control.
Samantha Benham-Hermetz, from Alzheimer’s Research UK, described the report’s findings as “groundbreaking”.
“Many people think of dementia as something that happens to people later in life, but dementia is not an inevitable part of ageing.”
