  Age is a fundamental parameter of stars, yet in many cases ages of individual
stars are presented without robust estimates of the uncertainty. We have
developed a Bayesian framework, BAFFLES, to produce the age posterior for a
star from its calcium emission strength (log($R'_{HK}$)) or lithium abundance
(Li EW) and $B-V$ color. We empirically determine the likelihood functions for
calcium and lithium as functions of age from literature measurements of stars
in benchmark clusters with well-determined ages. We use a uniform prior on age
which reflects a uniform star formation rate. The age posteriors we derive for
several test cases are consistent with literature ages found from other
methods. BAFFLES represents a robust method to determine the age posterior
probability distribution for any field star with $0.45 \leq B-V \leq 0.9$ and a
measurement of $R'_{HK}$ and/or $0.35 \leq B-V \leq 1.9$ and measured Li EW. We
compile colors, $R'_{HK}$, and Li EW from over 2630 nearby field stars from the
literature and present the derived BAFFLES age posterior for each star.
