An excerpt from a recent New York Times article :
These findings suggest that weight training can beneficially change the structure of the brain, but that “a minimum threshold of exercise needs to be achieved,” Dr. Liu-Ambrose said.
Visiting the gym once per week is probably insufficient. But twice per week may suffice......
Whatever the reason, exercise, including weight training, clearly “has benefit for the brain,” Dr. Liu-Ambrose said. “However we are just really now gaining an appreciation for how impactful exercise can be.”