  In this paper, we adopt the fluid limits to analyze Age of Information (AoI)
in a wireless multiaccess network with many users. We consider the case wherein
users have heterogeneous i.i.d. channel conditions and the statuses are
generate-at-will. Convergence of the AoI occupancy measure to the fluid limit,
represented by a Partial Derivative Equation (PDE), is proved within an
approximation error inversely proportional to the number of users. Global
convergence to the equilibrium of the PDE, i.e., stationary AoI distribution,
is also proved. Based on this framework, it is shown that an existing AoI lower
bound in the literature is in fact asymptotically tight, and a simple threshold
policy, with the thresholds explicitly derived, achieves the optimum
asymptotically. The proposed threshold-based policy is also much easier to
decentralize than the widely-known index-based policies which require comparing
user indices. To showcase the usability of the framework, we also use it to
analyze the average non-linear AoI functions (with power and logarithm forms)
in wireless networks. Again, explicit optimal threshold-based policies are
derived, and average age functions proven. Simulation results show that even
when the number of users is limited, e.g., $10$, the proposed policy and
analysis are still effective.
