"[A]lthough no healthy person knows how long he or she will live, retiring early increases the expected length of retirement and the number of years we spend in retirement is the greatest unpredictable determinant of retirement cost.... [R]etiring early ends savings contributions when they will typically be greatest.... [R]etiring early means drawing down savings until Social Security benefits or pension income kicks in. This places additional stress on savings early in retirement that increases the risk of outliving your savings."