  We present high-precision V, B-V color-magnitude diagrams (CMDs) for the
classic second parameter globular clusters M3 and M13 from wide-field deep CCD
photometry. The data for the two clusters were obtained during the same
photometric nights with the same instrument, allowing us to determine accurate
relative ages. Based on a differential comparison of the CMDs using the Delta
(B-V) method, an age difference of 1.7 +/- 0.7 Gyr is obtained between these
two clusters. We compare this result with our updated horizontal-branch (HB)
population models, which confirm that the observed age difference can produce
the difference in HB morphology between the clusters. This provides further
evidence that age is the dominant second parameter that influences HB
morphology.
