Age of Information is a measure of the freshness of status updates in monitoring applications and update-based systems. We study a real-time scenario where the sender (e.g. a sensor, in a remote sensing application) is limited 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 infinite battery and unit battery cases under a memoryless energy arrival process. Somewhat surprisingly, for any finite arrival rate of energy, there is a positive age threshold for transmission, corresponding to transmitting at a rate lower than that dictated by the rate of energy arrival. A lower bound on the average age is obtained for general battery size.