  Star-to-star light-element abundance variations, know as multiple stellar
populations (MPs), are common in almost all Galactic globular clusters.
Recently, MPs have also been detected in a number of massive clusters with ages
in excess of 2 Gyr in the Large Magellanic Cloud (LMC), thus indicating that
age is likely a control parameter for the presence of MPs. However, to
conclusively confirm this notion, additional studies of star clusters in the
LMC's `age gap' of 3--6 Gyr are required. Here, we use {\sl Hubble Space
Telescope} observations to study the 3 Gyr-old cluster NGC 2121. Compared with
so-called `simple' stellar population models, the cluster's red-giant branch
exhibits an apparent spread in a specific color index that is sensitive to
intrinsic chemical spreads. The observed spread can be explained by an
intrinsic spread in nitrogen abundance of $\sim$0.5--1.0 dex}. NGC 2121 has a
comparable mass to its younger counterparts without MPs, indicating that
cluster mass might not be the dominant parameter controlling the presence of
MPs. The transition phase between the occurrence of clusters with or without
MPs seems to occur at ages of 2--3 Gyr, indicating that cluster age may play a
dominant role in the establishment of MPs.
