This paper uses the Age of Information (AoI) to investigate the information freshness of non-orthogonal multiple access (NOMA) with successive interference cancellation (SIC). We consider two sensors sending update packets to a common access point. The SIC decoder exploits the different powers of the signal received from the sensors to separate their signals, e.g., the signal from the strong sensor is decoded first, followed by the weak sensor. A key issue in analyzing the AoI of SIC-based NOMA is that the weak sensor needs to wait for the successful decoding of the strong sensor before decoding its own packet. In particular, the information rates of the two sensors in SIC-based NOMA may differ due to the different received signal powers, resulting in different packet durations. Therefore, one sensor may send more packets than the other sensor during the same period, and the SIC operation complicates the AoI analysis further. To this end, this paper puts forth algorithms based on different information rates of sensors to compute the average AoI of SIC-based NOMA. Numerical results show that when the received signal powers are significantly different, SIC-based NOMA outperforms orthogonal multiple access (OMA) schemes, such as time division multiple access (TDMA) and frequency division multiple access (FDMA), in terms of average AoI. Copyright © 2022 IEEE.