  We take a critical look at the lithium depletion boundary (LDB) technique
that has recently been used to derive the ages of open clusters. We identify
the sources of experimental and systematic error and show that the probable
errors are larger by approximately a factor two than presently claimed in the
literature. We then use the Pleiades LDB age and photometry in combination with
evolutionary models to define empirical colour-T_eff relations that can be
applied to younger clusters. We find that these relationships DO NOT produce
model isochrones that match the younger cluster data. We propose that this is
due either to systematic problems in the evolutionary models or an age
(gravity) sensitivity in the colour-T_eff relation which is not present in
published atmospheric models.
