"Use it or lost it". Nothing seems more apt than for the aging Bear who starts to notice how rapidly the aging process starts to take hold once one hits their late 40's. It takes a redoubled effort (or you need to get one in the first place) to commit to weight-training as a core means of simply preserving existing metabolism - and counter the rapid decline in muscle-mass that aging implies.
Having read over 100+ fitness, exercise, diet, etc books over the years (thanks to a dirt-cheap second-hand bookstore in the neighborhood) few really stand out. Not just as a reference for weight-training's value in anti-aging -- but TURNING BACK THE CLOCK holds its own as the single most definitive book on exactly what you need to do, what you ought to be eating. A great read.