  The present paper focuses on an IoT setting in which a large number of
devices generate time-stamped updates addressed to a common gateway. Medium
access is regulated following a grant-free approach, and the system aims at
maintaining an up-to-date knowledge at the receiver, measured through the
average network age of information (AoI). In this context, we propose a
variation of the irregular repetition slotted ALOHA (IRSA) protocol. The
scheme, referred to as age-threshold IRSA (AT-IRSA), leverages feedback
provided by the receiver to admit to the channel only devices whose AoI exceeds
a dynamically adapted target value. By means of detailed networks simulations,
as well as of a simple yet tight analytical approximation, we demonstrate that
the approach can more than halve the average network AoI compared to plain
IRSA, and offers notable improvements over feedback-based state-of-the-art
slotted ALOHA solutions recently proposed in the literature.
