  In this paper, we consider the average age minimization problem where a
central entity schedules M users among the N available users for transmission
over unreliable channels. It is well-known that obtaining the optimal policy,
in this case, is out of reach. Accordingly, the Whittle's index policy has been
suggested in earlier works as a heuristic for this problem. However, the
analysis of its performance remained elusive. In the sequel, we overcome these
difficulties and provide rigorous results on its asymptotic optimality in the
many-users regime. Specifically, we first establish its optimality in the
neighborhood of a specific system's state. Next, we extend our proof to the
global case under a recurrence assumption, which we verify numerically. These
findings showcase that the Whittle's index policy has analytically provable
optimality in the many-users regime for the AoI minimization problem. Finally,
numerical results that showcase its performance and corroborate our theoretical
findings are presented.
