Panel A): the age as a function of [Fe/H] for the cluster sample of An et al. (2009) represented by asterisks. The red dashed and blue solid lines are the field ages obtained using isochrones with and without diffusion, respectively (see Sect. 4). Panel B): same as panel A) but with asterisks representing the ages and [Fe/H] of Salaris & Weiss (2002). Panel C): same as panels A) and B), plotting this time the ages and [Fe/H] of Dotter et al. (2010) with asterisks. Panel D): again D10 clusters but compared with field ages using DSEP isochrones.