  The Mount Wilson Ca II index log$(R'_{\rm HK})$ is the accepted standard
metric of calibration for the chromospheric activity versus age relation of FGK
stars. Recent results claim its inability to discern activity levels, and thus
ages, for stars older than $\sim$2 Gyr, which would severely hamper its
application to date disk stars older than the Sun. We present a new
activity-age calibration of the Mt. Wilson index explicitly taking into account
mass and $[$Fe/H$]$ biases, implicit in samples of stars selected to have
precise ages, that have so far gone unappreciated. We show that these selection
biases tend to blur the activity-age relation for large age ranges. We
calibrate the Mt. Wilson index for a sample of field FGK stars with precise
ages, covering a wide range of mass and $[$Fe/H$]$, augmented with data from
the Pleiades, Hyades, M67 clusters, and the Ursa Major moving group. We further
test the calibration with extensive new Gemini/GMOS log$(R'_{\rm HK})$ data of
the old, solar $[$Fe/H$]$ clusters M67 and NGC 188. The observed NGC 188
activity level is clearly lower than M67. We correctly recover the isochronal
age of both clusters and establish the viability of deriving usable
chromospheric ages for solar-type stars up to at least $\sim$6 Gyr, average
errors being $\sim$0.14 dex provided that the mass and $[$Fe/H$]$ dimensions
are explicitly accounted for. We test our calibration against
asteroseismological ages, finding excellent correlation ($\rho$ = +0.89). We
show that our calibration improves the chromospheric age determination for a
wide range of ages, masses, and metallicities in comparison to previous
age-activity relations.
