Smoking and high blood pressure over the age of 50 both lead to accelerated mental decline, research has shown.
Scientists analysed risk factor data for more than 8,000 older adults taking part in the English Longitudinal Study of Ageing (Elsa).
Tests of memory, planning and overall mental ability were carried out after four and eight years.
The study showed that smoking consistently reduced all three performance measures after four years.
High blood pressure and high risk of stroke were associated with lower scores for memory and overall mental ability after eight years. Being overweight was linked to poor memory.
The findings appear in the journal Age and Ageing.
They indicate that future trials should focus on combinations of risk factors rather than individual causes of mental decline, say the researchers.
Lead scientist Dr Alex Dregan, from King's College London, said: "Cognitive decline becomes more common with ageing and for an increasing number of people interferes with daily functioning and well-being.
"Some older people can become forgetful, have trouble remembering common words or have problems organising daily tasks more than others.
"We have identified a number of risk factors which could be associated with accelerated cognitive decline, all of which, could be modifiable. This offers valuable knowledge for future prevention and treatment interventions."
The results indicate that high blood pressure has a gradual effect on the brain over a long period, according to the scientists.
This could explain why short-term trials of blood pressure lowering drugs being used to treat mental decline had failed to show a clear benefit.
Dr Dregan added: "Our research suggests that the most promising approach to delaying or preventing early ageing of the brain is one that acknowledges the multi-causality of cognitive decline."
Jessica Smith, from the Alzheimer's Society, said: "We all know smoking, a high blood pressure, high cholesterol levels and a high BMI (body mass index) is bad for our heart. This research adds to the huge amount of evidence that also suggests they can be bad for our head too.
"One in three people over 65 will develop dementia but there are things people can do to reduce their risk. Eating a balanced diet, maintaining a healthy weight, exercising regularly, getting your blood pressure and cholesterol checked and not smoking can all make a difference."
Dr Simon Ridley, from the charity Alzheimer's Research UK, said: "Research has repeatedly linked smoking and high blood pressure to a greater risk of cognitive decline and dementia, and this study adds further weight to that evidence. Cognitive decline as we age can develop into dementia, and unravelling the factors that are linked to this decline could be crucial for finding ways to prevent the condition."