Your risk is also increased by factors you can't control, such as whether you're female, of south east asian descent or whether someone in your family has had the condition.
Researchers at New York's Union College found older adults with mild cognitive impairment - often a precursor to Alzheimer's disease - showed significant improvement after playing video games that require physical exercise.
A team of United Kingdom researchers wrote in the British Medical Journal that they do not recommend moderate to high intensity exercise as a treatment option for dementia.
But whether or not it can slow symptoms after the onset of mental decline, has been the subject of much debate.
British Medical Journal (BMJ) research has found that exercise programmes fail to slow brain decline in people living with mild to moderate dementia.
Oxford University studied almost 500 people, with an average age of 77 years, in 15 regions across England who were randomly given either supervised exercise and support programmes, or normal elderly care.
They took part in 60-90-minute group sessions in a gym twice a week for four months, and home exercises for an additional hour per week.
In the exercise group, the decline was steeper, "however, the average difference was small and clinical relevance was uncertain", said a press statement.
"The exercise programme might possibly have worsened cognitive impairment".
The sessions included 20 minutes on a fixed cycle and lifting weights while getting out of a chair.
Lamb said this study should not stop people from taking people with dementia for walks, swimming or other sports, because low intensity workout is still beneficial to patients.
Currently, as a dementia therapy that does not involve medication, the NHS recommends group cognitive stimulation therapy classes, where sufferers undertake exercises created to improve their memory, problem-solving skills and language ability. But the authors said the benefits "do not translate into improvements in cognitive impairment, activities in daily living, behaviour, or health related quality of life".
"There is at present no cure for dementia".