  The conventional picture of coeval, chemically homogeneous, populous star
clusters -- known as `simple stellar populations' (SSPs) -- is a view of the
past. Photometric and spectroscopic studies reveal that almost all ancient
globular clusters in the Milky Way and our neighbouring galaxies exhibit
star-to-star light-element abundance variations, typically known as 'multiple
populations' (MPs). Here, we analyse photometric $\it Hubble$ $\it Space$ $\it
Telescope$ observations of three young ($<$2 Gyr-old) Large and Small
Magellanic Cloud clusters, NGC 411, NGC 1718 and NGC 2213. We measure the
widths of their red-giant branches (RGBs). For NGC 411, we also use a
pseudo-colour--magnitude diagram (pseudo-CMD) to assess its RGB for evidence of
MPs. We compare the morphologies of the clusters' RGBs with artificially
generated SSPs. We conclude that their RGBs do not show evidence of significant
broadening beyond intrinsic photometric scatter, suggesting an absence of
significant chemical abundance variations in our sample clusters. Specifically,
for NGC 411, NGC 1718 and NGC 2213 we derive maximum helium-abundance
variations of delta_Y=0.003$\pm$0.001 Y=0.300), 0.002$\pm$0.001 (Y=0.350) and
0.004$\pm$0.002 (Y=0.300), respectively. We determined an upper limit to the
NGC 411 nitrogen-abundance variation of $\Delta$[N/Fe] = 0.3 dex; the available
data for our other clusters do not allow us to determine useful upper limits.
It thus appears that the transition from SSPs to MPs occurs at an age of ~2
Gyr, implying that age might play an important role in this transition. This
raises the question as to whether this is indeed a fundamental minimum-age
limit for the formation of MPs.
