  Using age of information as the freshness metric, we examine a multicast
network in which real-time status updates are generated by the source and sent
to a group of $n$ interested receivers. We show that in order to keep the
information freshness at each receiver, the source should terminate the
transmission of the current update and start sending a new update packet as
soon as it receives the acknowledgements back from any $k$ out of $n$ nodes. As
the source stopping threshold $k$ increases, a node is more likely to get the
latest generated update, but the age of the most recent update is more likely
to become outdated. We derive the age minimized stopping threshold $k$ that
balances the likelihood of getting the latest update and the freshness of the
latest update for shifted exponential link delay. Through numerical evaluations
for different stopping strategies, we find that waiting for the
acknowledgements from the earliest $k$ out of $n$ nodes leads to lower average
age than waiting for a pre-selected group of $k$ nodes. We also observe that a
properly chosen threshold $k$ can prevent information staleness for increasing
number of nodes $n$ in the multicast network.
