We consider a system where multiple terminals transmit their randomly generated status updates to a base station (BS) sharing a wireless multiaccess uplink channel. The problem of interest, especially in massive Internet-of-Things systems, is that how to schedule the terminals to minimize the time-average age-of-information in a decentralized manner, namely terminals transmit autonomously, without signalling exchange (overhead) with the BS or other terminals. Towards this end, the round-robin with one-packet buffers (the newest packet at each terminal only) policy (RR-ONE) is proposed, and proved optimal among arrival-independent renewal (AIR) policies. In addition to its simple structure which is instrumental for decentralized implementation, RR-ONE is further proved asymptotically (massive terminals) optimal among all policies, including centralized and non-causal policies.