The information sampling and transmission scheduling problems have been treated separately in the context of real-time status update, or age of information optimization. In this paper, a unified sampling and scheduling ($S^2$) approach is proposed, focusing on decentralized updates in multiaccess wireless networks. To gain some insights, we first analyze an example consisting of two-state Markov sources, showing that when both optimized, the unified approach outperforms the decoupled approach significantly in terms of status tracking error, by capturing the key status variation. We then generalize to source nodes with random-walk state transitions whose scaling limit is Wiener processes, the closed-form Whittle’s index with arbitrary status tracking error functions is obtained and indexability established. Furthermore, a mean-field approach is introduced to solve the decentralized status update design explicitly. In addition to simulation results which validate the optimality of the proposed $S^2$ scheme and its advantage over the decoupled approach, a use case of dynamic channel state information (CSI) update, with CSI generated by a ray-tracing simulator, is investigated.