Freshness of status update packets is essential for wide range of real-time Internet of things applications. In this paper, we study the information freshness of a single-server multi-source queueing model under a first-come first-served (FCFS) serving policy. In the considered model, each source independently generates status update packets according to a Poisson process. The information freshness of the status updates of each source is evaluated by the average age of information (AoI). We derive an approximate expression for the average AoI for a multi-source M/G/1 queueing model having a general service time distribution. Simulation results are provided to validate and assess the tightness of the proposed approximate expression for the average AoI in the M/G/1 queueing model where the service time follows a gamma distribution.