  We examine lossless data compression from an average delay perspective. An
encoder receives input symbols one per unit time from an i.i.d. source and
submits binary codewords to a FIFO buffer that transmits bits at a fixed rate
to a receiver/decoder. Each input symbol at the encoder is viewed as a status
update by the source and the system performance is characterized by the status
update age, defined as the number of time units (symbols) the decoder output
lags behind the encoder input. An upper bound on the average status age is
derived from the exponential bound on the probability of error in streaming
source coding with delay. Apart from the influence of the error exponent that
describes the convergence of the error, this upper bound also scales with the
constant multiplier term in the error probability. However, the error exponent
does not lead to an accurate description of the status age for small delay and
small blocklength. An age optimal block coding scheme is proposed based on an
approximation of the average age by converting the streaming source coding
system into a D/G/1 queue. We compare this scheme to the error exponent optimal
coding scheme which uses the method of types. We show that maximizing the error
exponent is not equivalent to minimizing the average status age.
