For proliferating cells subject to both division and death, how can one estimate the average
generation number of the living population without continuous observation or a division-diluting
dye? In this paper we provide a method for cell systems such that at each division there is an
unlikely, heritable one-way label change that has no impact other than to serve as a distinguishing
marker. If the probability of label change per cell generation can be determined and the proportion
of labeled cells at a given time point can be measured, we establish that the average generation
number of living cells can be estimated. Crucially, the estimator does not depend on knowledge
of the statistics of cell cycle, death rates or total cell numbers. We validate the estimator and
illustrate its features through comparison with published data and physiologically parameterized
stochastic simulations, using it to suggest new experimental designs
