The Image Reduction and Analysis Facility ( IRAF) is distributed by the National Optical Astronomy Observatories, which is operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the US National Science Foundation. STSDAS, the Space Telescope Science Data Analysis System, contains tasks complementary to the existing IRAF tasks. We used Version 3.5 (March 2006) for the data reduction performed in this paper.

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The Interactive Data Language (IDL) is licensed by Research Systems Inc., of Boulder, CO, USA.

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... tool

Available from http://stev.oapd.inaf.it/cmd

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... law

See http://stev.oapd.inaf.it/cmd

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... estimates

It is well known that broad-band SED fitting results in artefacts in the cluster age distribution. This is predominantly caused by specific features in the SSP models, such as the onset and presence of red giant branch or asymptotic giant branch stars at, respectively, $\sim$ 10 and $\sim$ 100 Myr (e.g., Bastian et al. 2005). In addition, the peak in Fig. 9 at an age of $\sim$ 10 Myr may be an artefact caused by the rapid evolution of stellar populations around $\sim$ 10-30 Myr (e.g., Lee et al. 2005).

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... evidence

The apparent peak in Fig. 9 at an age of $\sim$ 10 Myr is most likely due to the residual effects of (i) the age-extinction degeneracy (which is particularly important at these young ages), and (ii) the fact that the youngest isochrone included in the Analy SED tool is of an age of $2.5 \times 10^6$ yr, with limited age resolution at these ages. In addition, we remind the reader that we have adopted a fixed, solar metallicity, which may also give rise to a small age-metallicity degeneracy (in view of the variability of the galactic background and the associated photometric uncertainties, it is impractical to introduce an additional free parameter in the fits; the metallicity is expected to be least variable of our choice of possible free parameters). The consequence of these effects is that the youngest clusters tend to accumulate close to the lower age cut-off, and hence this peak should be taken with extreme caution (see, e.g., de Grijs et al. 2003a, for a detailed analysis).

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