"Physical activity may be beneficial for persons with dementia. But due to the small number of studies we have not been able to demonstrate this," said review lead author Dorothy Forbes, an associate professor with the faculty of health sciences at the University of Western Ontario, in Canada.
On the other hand, there is no evidence that physical activity is harmful either, she said.
Estimates indicate that dementia - known in the past as senility - affects about 14 percent of Americans ages 71 and older. More than one-third of Americans over 90 likely suffer from the condition, which causes forgetfulness, confusion and muddled thinking.
A variety of medical conditions causes dementia, including Alzheimer's disease and stroke.
Physicians are unlikely to prescribe physical activity as a treatment for dementia patients because there is little evidence to support its value, Forbes said.
Nevertheless, exercise is not impossible for many dementia patients, who might be able to walk, swim and exercise in groups with assistance, she said.
Researchers have shown that exercise can improve cognition and mental health in older adults, and some studies suggest that it could delay dementia from three to six years or reduce the risk that patients will develop cognitive problems, Forbes said.
"It is less clear if physical activity manages or improves other symptoms among persons with a diagnosis of dementia," she said.
The review appears in the latest issue of The Cochrane Library, a publication of The Cochrane Collaboration, an international organization that evaluates medical research.
While evidence from animal research indicates that physical activity could be a useful treatment for dementia, the review authors only found four studies that examined the effects of exercise in humans.
Two of the studies were not included in the analysis because the reviewers could not get details from the original study authors.
Both of the remaining studies were small and only included Alzheimer disease patients. One looked at just 11 patients; the other examined 134, but many of those did not complete their exercise regimens.
Still, the researchers in the latter study found that those who did exercise seemed to do better at handling the tasks of daily living.
While there is little research supporting physical activity as a helpful treatment for elderly people with dementia, there is no evidence that it is harmful, Forbes said.