LAGUNA WOODS, Calif. — The ladies in the card room are playing bridge, and at their age the game is no hobby. It is a way of life, a daily comfort and challenge, the last communal campfire before all goes dark. “We play for blood,” says Ruth Cummins, 92, before taking a sip of Red Bull at a recent game. “It’s what keeps us going,” adds Georgia Scott, 99. “It’s where our closest friends are.” In recent years scientists have become intensely interested in what could be called a super memory club — the fewer than one in 200 of...