  While the strong anti-correlation between chromospheric activity and age has
led to the common use of the Ca II H & K emission index (R'_HK = L_HK/L_bol) as
an empirical age estimator for solar type dwarfs, existing activity-age
relations produce implausible ages at both high and low activity levels. We
have compiled R'_HK data from the literature for young stellar clusters, richly
populating for the first time the young end of the activity-age relation.
Combining the cluster activity data with modern cluster age estimates, and
analyzing the color-dependence of the chromospheric activity age index, we
derive an improved activity-age calibration for F7-K2 dwarfs (0.5 < B-V < 0.9
mag). We also present a more fundamentally motivated activity-age calibration
that relies on conversion of R'_HK values through the Rossby number to rotation
periods, and then makes use of improved gyrochronology relations. We
demonstrate that our new activity-age calibration has typical age precision of
~0.2 dex for normal solar-type dwarfs aged between the Hyades and the Sun
(~0.6-4.5 Gyr). Inferring ages through activity-rotation-age relations accounts
for some color-dependent effects, and systematically improves the age estimates
(albeit only slightly). We demonstrate that coronal activity as measured
through the fractional X-ray luminosity (R_X = L_X/L_bol) has nearly the same
age- and rotation-inferring capability as chromospheric activity measured
through R'_HK. As a first application of our calibrations, we provide new
activity-derived age estimates for the nearest 100 solar-type field dwarfs (d <
15 pc).
