  Stellar population models are commonly calculated using star clusters as
calibrators for those evolutionary stages that depend on free parameters.
However, discrepancies exist among different models, even if similar sets of
calibration clusters are used. With the aim of understanding these
discrepancies, and of improving the calibration procedure, we consider a set of
43 Magellanic Cloud (MC) clusters taking age and photometric information from
the literature. We carefully assign ages to each cluster based on up-to-date
determinations ensuring that these are as homogeneous as possible. To cope with
statistical fluctuations, we stack the clusters in five age bins deriving for
each of them integrated luminosities and colors. We find that clusters become
abruptly red in optical and optical-IR colors as they age from ~0.6 to 1 Gyr,
which we interpret as due to the development of a well-populated thermally
pulsing asymptotic giant branch (TP-AGB). We argue that other studies missed
this detection because of coarser age binnings. Maraston (2005; M05) and
Girardi et al. (2010) models predict the presence of a populated TP-AGB at ~0.6
Gyr, with a correspondingly very red integrated color, at variance with the
data; Bruzual & Charlot (2003) and Conroy et al. (2009) models run within the
error bars at all ages. The discrepancy between the synthetic colors of M05
models and the average colors of MC clusters results from the now obsolete age
scale adopted. Finally, our finding that the TP-AGB phase appears to develop
between ~0.6-1 Gyr is dependent on the adopted age scale for the clusters and
may have important implications for stellar evolution.
