  We estimate cluster ages from lithium depletion in five pre-main-sequence
groups found within 100 pc of the Sun: TW Hydrae Association, Eta Chamaeleontis
Cluster, Beta Pictoris Moving Group, Tucanae-Horologium Association and AB
Doradus Moving Group. We determine surface gravities, effective temperatures
and lithium abundances for over 900 spectra through least squares fitting to
model-atmosphere spectra. For each group, we compare the dependence of lithium
abundance on temperature with isochrones from pre-main-sequence evolutionary
tracks to obtain model dependent ages. We find that the Eta Chamaelontis
Cluster and the TW Hydrae Association are the youngest, with ages of 12+/-6 Myr
and 12+/-8 Myr, respectively, followed by the Beta Pictoris Moving Group at
21+/-9 Myr, the Tucanae-Horologium Association at 27+/-11 Myr, and the AB
Doradus Moving Group at an age of at least 45 Myr (where we can only set a
lower limit since the models -- unlike real stars -- do not show much lithium
depletion beyond this age). Here, the ordering is robust, but the precise ages
depend on our choice of both atmospheric and evolutionary models. As a result,
while our ages are consistent with estimates based on Hertzsprung-Russell
isochrone fitting and dynamical expansion, they are not yet more precise. Our
observations do show that with improved models, much stronger constraints
should be feasible: the intrinsic uncertainties, as measured from the scatter
between measurements from different spectra of the same star, are very low:
around 10 K in effective temperature, 0.05 dex in surface gravity, and 0.03 dex
in lithium abundance.
