  We investigate the use of the gravity sensitive neutral sodium (NaI) doublet
at 8183 Angstroms 8195 Angstroms (Na 8200 Angstrom doublet) as an age indicator
for M dwarfs. We measured the Na doublet equivalent width (EW) in giants, old
dwarfs, young dwarfs, and candidate members of the Beta Pic moving group using
medium resolution spectra. Our Na 8200 Angstrom doublet EW analysis shows that
the feature is useful as an approximate age indicator in M-type dwarfs with
(V-K_s) >= 5.0, reliably distinguishing stars older and younger than 100 Myr. A
simple derivation of the dependence of the Na EW on temperature and gravity
supports the observational results. An analysis of the effects of metallicity
show that this youth indicator is best used on samples with similar
metallicity. The age estimation technique presented here becomes useful in a
mass regime where traditional youth indicators are increasingly less reliable,
is applicable to other alkali lines, and will help identify new-low mass
members in other young clusters and associations.
