  Age of information (AoI) is gaining attention as a valuable performance
metric for many IoT systems, in which a large number of devices report
time-stamped updates to a central gateway. This is the case, for instance, of
remote sensing, monitoring, or tracking, with broad applications in the
industrial, vehicular, and environmental domain. In these settings, AoI
provides insights that are complementary to those offered by throughput or
latency, capturing the ability of the system to maintain an up-to-date view of
the status of each transmitting device. From this standpoint, while a good
understanding of the metric has been reached for point-to-point links,
relatively little attention has been devoted to the impact that link layer
solutions employed in IoT systems may have on AoI. In particular, no result is
available for modern random access protocols, which have recently emerged as
promising solutions to support massive machine-type communications. To start
addressing this gap we provide in this paper the first study of the AoI of a
scheme in this family, namely irregular repetition slotted ALOHA (IRSA). By
means of a Markovian analysis, we track the AoI evolution at the gateway, prove
that the process is ergodic, and derive a compact closed form expression for
its stationary distribution. Leaning on this, we compute exact formulations for
the average AoI and the age violation probability. The study reveals
non-trivial design trade-offs for IRSA and highlights the key role played by
the protocol operating frame size. Moreover, a comparison with the performance
of a simpler slotted ALOHA strategy highlights a remarkable potential for
modern random access schemes in terms of information freshness.
