Age–metallicity relation for clusters studied in this work. Clusters with ages derived using Darthmouth isochrones (black squares); clusters with reliable number of stars for metallicity determination (open squares), having ages derived from the Padova isochrones; clusters with 1−4 stars for metallicity calculation with the Padova ages (open circles). Red dots indicate clusters whose metallicity was determined for the first time. Overplotted theoretical models: PT98 bursting model (solid line); closed box model for −0.6 dex and −0.5 dex (dashed and dash-dotted line, respectively); HZ04 model (dotted line); C13-B and C13-C models (blue dashed and dotted lines, respectively); TB09-1, TB09-2, and TB09-3 models (magenta dotted, dashed, and solid lines, respectively).