  Age of information is a new concept that characterizes the freshness of
information at end devices. This paper studies the age of information from a
scheduling perspective. We consider a wireless broadcast network where a
base-station updates many users on stochastic information arrivals. Suppose
that only one user can be updated for each time. In this context, we aim at
developing a transmission scheduling algorithm for minimizing the long-run
average age. To develop a low-complexity transmission scheduling algorithm, we
apply the Whittle's framework for restless bandits. We successfully derive the
Whittle index in a closed form and establish the indexability. Based on the
Whittle index, we propose a scheduling algorithm, while experimentally showing
that it closely approximates an age-optimal scheduling algorithm.
