The recent discovery of multiple stellar populations in massive Galactic globular clusters poses a serious challenge for models of star cluster formation and evolution. An unique angle on this problem is being provided by intermediate-age clusters in the Magellanic Clouds with peculiar main-sequence turn-off morphologies. These clusters are young enough to allow us to measure age differences of the order of 100 Myr, as the expected age for the different stellar generations according to the most popular theories of their origin. Recent discoveries, based on ACS data of unparalleled photometric accuracy, have demonstrated that the CMDs of a large fraction of these clusters { 70%} are not consistent with the simple, single stellar population hypothesis.The conditions under which massive Galactic globular clusters experience the formation of multiple generations remain controversial, and even more so within intermediate-age star clusters. To properly constraint the multi-population phenomenon in Magellanic Cloud star clusters, we propose deep UV/IR imaging of NGC 1846, a star cluster where multiple populations are already identified.The proposed observation will allow us to accurately measure the age difference between the stellar populations providing fundamental clues on the formation mechanism, and discover possible main sequence splitting due to different He contents. Our simulations of WFC3 performance suggest that we will be able to detect main sequence splitting caused by He differences as small as DeltaY=0.02.;