Activities that engage the brain holistically are key in protecting the attention, memory and learning parts of the brain.
It's really more than just doing mentally-stimulating exercises.
MORE THAN JUST
Physical activities are described as the "single, strongest factor" known to be beneficial for protecting the brain.
and the most doable activity is
IT STARTS WITH A SINGLE STEP!
RADIOLOGICAL SOCIETY OF NORTH AMERICA
A study done by RSNA, amongst others, found that taking regular walks (and starting early) can help delay the onset and/or progression of dementia.