Doing aerobic and resistance training several times a week is the best way the over 50s can keep their mind sharp.

Taking up exercise at any age is worthwhile for the brain, Australian research shows.

A study published in the British Medical Journal of Sports Medicine has found exercising for at least 45 minutes several times a week can boost brain power in people aged over 50.

This is regardless of their current state of brain health, say the authors.

Aerobic exercise such as swimming, cycling and jogging; resistance training including weights, combined aerobic and weight training and tai chi are all "similarly effective".

Physical exercise is seen as a promising intervention to prevent or delay dementia in older adults, yet the evidence has been inconclusive.

Researchers at the University of Canberra and ANU say the evidence is now strong enough to recommend prescribing both aerobic and resistance training to improve brain health in the over 50s.

Analysis of 39 studies found several types of exercise help improve thinking, attention, memory and executive function.

Aerobic exercise significantly enhanced cognitive abilities, while resistance training had a pronounced effect on executive function, memory, and working memory.

"This study confirms previous suggestions that resistance training may play an important role in improving cognitive function in older adults," wrote the authors.

"Although this does not show that resistance training is better than other modes of exercise, it does suggest that this type of training has particularly pronounced effects on these domains of cognitive function," they added.

Thai chi also improved cognitive function but the authors did note the small number of studies on this type of exercise included in the study.

Nevertheless, they suggest, this is an important finding because non-traditional modes of exercise, such as tai chi, may be suitable for those unable to do more challenging physical activity.

In terms of how much and how often, a exercise session lasting between 45 and 60 minutes of moderate to vigorous intensity, and of any frequency, was good for brain health.

"Our meta-analysis provides positive evidence for the prescription of both aerobic and resistance training, in accordance with exercise recommendations, for this age group to specifically improve cognitive functions," the authors concluded.