  Current spectroscopic surveys are producing large catalogs of chemical
abundances for stars of all types. The yttrium to magnesium ratio, [Y/Mg], has
emerged as a candidate age indicator for solar twins in the local stellar
neighborhood. However, it is unclear whether it is a viable age diagnostic for
more diverse stellar types, so we investigate [Y/Mg] as an age indicator for
the FGK-type planet host stars observed by $Kepler$. We find that the [Y/Mg]
"Clock" is most precise for solar twins, with a [Y/Mg]/Age slope of $m$ =
$-$0.0370 $\pm$ 0.0071 dex/Gyr and $\sigma_{\mathrm{Age}}$ = 2.6 Gyr. We
attribute the lower precision compared to literature results to non-solar twins
contaminating our solar twin sample and recommend a 1.5 Gyr systematic
uncertainty for stellar ages derived with any [Y/Mg]-Age relation. We also
analyzed the [Y/Mg] Clock as a function of $T_{\mathrm{eff}}$, $\log g$, and
metallicity individually and find no strong trends, but compute statistically
significant [Y/Mg]-Age relations for subsamples defined by ranges in
$T_{\mathrm{eff}}$, $\log g$, and metallicity. Finally, we compare [Y/Mg] and
rotation ages and find statistically similar trends as for isochrone ages,
although we find that rotation ages perform better for GK dwarfs while
isochrones perform better for FG subgiants. We conclude that the [Y/Mg] Clock
is most precise for solar twins and analogs but is also a useful age diagnostic
for FGK stars.
