A study done by the British Journal of Medicine showed that exercise in those aged 50 and above was beneficial for cognitive fitness. No matter what their cognitive status was, any form of exercise showed improvement. All forms of exercise were studied: aerobic, resistance, yoga, tai chi, etc. all helped. The longer (45-60 minute interval) and more intense showed more improvement that milder forms of exercise.
Here’s the whole study: https://www.ncbi.nlm.nih.gov/pubmed/28438770