  We present the results of photometric measurements from images of the LMC
cluster ESO 121-SC03 taken with HST/ACS. Our resulting CMD reaches 3 magnitudes
below the main-sequence turn-off, representing the deepest observation of this
cluster to date. We also present similar photometry from ACS imaging of the
accreted Sagittarius dSph cluster Pal. 12, used in this work as a comparison
cluster. From analysis of its CMD, we obtain estimates for the metallicity and
reddening of ESO 121-SC03: [Fe/H] = -0.97 +/- 0.10 and E(V-I) = 0.04 +/- 0.02,
in excellent agreement with previous studies. The observed horizontal branch
level in ESO 121-SC03 suggests this cluster may lie 20 per cent closer to us
than does the centre of the LMC. ESO 121-SC03 also possesses a significant
population of blue stragglers, which we briefly discuss. Our new photometry
allows us to undertake a detailed study of the age of ESO 121-SC03 relative to
Pal. 12 and the Galactic globular cluster 47 Tuc. Our main result is that ESO
121-SC03 is 73 +/- 4 per cent the age of 47 Tuc, and 91 +/- 5 per cent the age
of Pal. 12. Pal. 12 is 79 +/- 6 per cent as old as 47 Tuc, consistent with
previous work. Our result corresponds to an absolute age for ESO 121-SC03 in
the range 8.3-9.8 Gyr depending on the age assumed for 47 Tuc, therefore
confirming ESO 121-SC03 as the only known cluster to lie squarely within the
LMC age gap. We briefly discuss a suggestion from earlier work that ESO
121-SC03 may have been accreted into the LMC system.
