Dr. Snowdon said in his book that when people ask him what they can do to help their children develop healthy brains, he simply says, "Read to them." His studies have shown that there's something about reading to small children that helps their brain cells develop.
He also said that mental activity throughout life can help to preserve and enhance brain cells, too. It's not foolproof, since some very intelligent and creative people developed dementia later in life. I'm thinking, for example, of the gifted Jesuit Fr. Bernard Lonergan. When he realized that his mind was starting to fail, he accepted it in a spirit of faith and offered it as a sacrifice to the Lord.