  Sensor sources submit updates to a monitor through an unslotted,
uncoordinated, unreliable multiple access collision channel. The channel is
unreliable; a collision-free transmission is received successfully at the
monitor with some transmission success probability. For an infinite-user model
in which the sensors collectively transmit updates as a Poisson process and
each update has an independent exponential transmission time, a stochastic
hybrid system (SHS) approach is used to derive the average age of information
(AoI) as a function of the offered load and the transmission success
probability. The analysis is then extended to evaluate the individual age of a
selected source. When the number of sources and update transmission rate grow
large in fixed proportion, the limiting asymptotic individual age is shown to
provide an accurate individual age approximation for a small number of sources.
