**Figure 5.** **Theoretical trajectories (solid curves) of old-age (80-110) mortality dynamics for a heterogeneous population composed of two subpopulations.** Variations in relative sizes of the subpopulations permit the reproduction of all three observations for late-life mortality: deceleration, plateau and decline. Once the individuals of the frailest subpopulation die out, the mortality of the entire population follows the exponential dynamics of the most robust subpopulation. In panel (**A**) the same subpopulation remains frailest over all ages, while in panel (**B**) the subpopulation which is frailest before age 90 becomes the most robust after age 90. Note that the plots are shown on a semi-logarithmic scale.