Freshness of status update packets is essential for enabling a wide range of Internet of Things (IoT) applications. In this paper, we consider a status update system in which various sensors are assigned to transmit status update packets of a physical process to a desired destination. We consider that the sensors share a wireless channel and contend for the channel access based on a carrier sense multiple access (CSMA) method. We study freshness of the status update system at the destination using the age of information (AoI) metric. To this end, we analyze the worst case average AoI for each sensor in the CSMA-based system. Numerical results show that the AoI in the CSMA-based system may dramatically increase when the number of sensors increases. Moreover, we observe that the contention window size and the packet arrival rate must be optimized since they have a critical role in the performance of the system.