  Age of Information is a measure of the freshness of status updates in
monitoring applications and update-based systems. We study a real-time remote
sensing scenario with a sensor which is restricted by time-varying energy
constraints and battery limitations. The sensor sends updates over a packet
erasure channel with no feedback. The problem of finding an age-optimal
threshold policy, with the transmission threshold being a function of the
energy state and the estimated current age, is formulated. The average age is
analyzed for the unit battery scenario under a memoryless energy arrival
process. Somewhat surprisingly, for any finite arrival rate of energy, there is
a positive age threshold for transmission, which corresponding to transmitting
at a rate lower than that dictated by the rate of energy arrivals. A lower
bound on the average age is obtained for general battery size.
