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The Mount Wilson Ca II index log$(R'_{\rm HK})$ is the accepted standardmetric of calibration for the chromospheric activity versus age relation of FGKstars. Recent results claim its inability to discern activity levels, and thusages, for stars older than $\sim$2 Gyr, which would severely hamper itsapplication to date disk stars older than the Sun. We present a newactivity-age calibration of the Mt. Wilson index explicitly taking into accountmass and $[$Fe/H$]$ biases, implicit in samples of stars selected to haveprecise ages, that have so far gone unappreciated. We show that these selectionbiases tend to blur the activity-age relation for large age ranges. Wecalibrate the Mt. Wilson index for a sample of field FGK stars with preciseages, covering a wide range of mass and $[$Fe/H$]$, augmented with data fromthe Pleiades, Hyades, M67 clusters, and the Ursa Major moving group. We furthertest the calibration with extensive new Gemini/GMOS log$(R'_{\rm HK})$ data ofthe old, solar $[$Fe/H$]$ clusters M67 and NGC 188. The observed NGC 188activity level is clearly lower than M67. We correctly recover the isochronalage of both clusters and establish the viability of deriving usablechromospheric ages for solar-type stars up to at least $\sim$6 Gyr, averageerrors being $\sim$0.14 dex provided that the mass and $[$Fe/H$]$ dimensionsare explicitly accounted for. We test our calibration againstasteroseismological ages, finding excellent correlation ($\rho$ = +0.89). Weshow that our calibration improves the chromospheric age determination for awide range of ages, masses, and metallicities in comparison to previousage-activity relations.

The age-mass-metallicity-activity relation for solar-type stars: Comparisons with asteroseismology and the NGC 188 open cluster