  We consider a wireless sensor network in which all the nodes wish to spread
their updates over the network using CSMA/CA protocol. We investigate the age
of information of the spreading process from a transmitter perspective, which
is referred to as the \textit{broadcast age of information (BAoI)}. To be
specific, BAoI is the age of the latest update successfully broadcasted to the
one-hop neighbors of a node, and thus is suitable to measure the rapidity of
the update spreading process. We establish an equivalent transmission model of
the network by deriving the transmission probability and the collision
probability of nodes. With this equivalent model, we then present the average
BAoI of the network explicitly. Our results present the scaling laws of average
BAoI with respect to node density and frame length, and are further illustrated
through numerical results. As is shown, the average BAoI is increasing with
node density and is convex in frame length, i.e., would be large when frame
length is very small or very large.
