The world’s population is growing older, taking us into uncharted demographic waters. By 2050, over one-fifth of the US population will be 65 or older, up from the current figure of one-seventh. The number of centenarians worldwide will double by 2023 and double again by 2035. Projections suggest life expectancy will surpass 100 in some industrialized countries by the second half of this century — roughly triple the lifespan that prevailed worldwide throughout most of human history.