  We have constrained the age of the globular cluster S312 in the Andromeda
galaxy (M31) by comparing its multicolor photometry with theoretical stellar
population synthesis models. This is both a check on the age of this globular
cluster, as well a check on our methodology. Main-sequence photometry has been
the most direct method for determining the age of a star cluster. S312 was
observed as part of the Beijing-Arizona-Taiwan-Connecticut (BATC) Multicolor
Sky Survey from 1995 February to 2003 December. The photometry of BATC images
for S312 was taken with 9 intermediate-band filters covering 5000-10000\AA.
Combined with photometry in the near-ultraviolet (NUV) of {\sl GALEX},
broad-band $UBVR$ and infrared $JHK_s$ of 2MASS, we obtained the accurate
spectral energy distributions (SEDs) of S312 from 2267-20000\AA. A quantitative
comparison to simple stellar population models yields an age of
$9.5_{-0.99}^{+1.15}$ Gyr, which is in very good agreement with the previous
determination by main-sequence photometry. S312 has a mass of
$9.8\pm{1.85}\times 10^5 \rm M_\odot$, and is a medium-mass globular cluster in
M31. By analysis of errors of ages determined based on the SED fitting method
of this paper, secure age constraints are derived with errors of $< 3$ Gyr for
ages younger than 9 Gyr. In fact, the theoretical SEDs are not sensitive to the
variation of age for ages greater than $\sim 10$ Gyr. Therefore, for globular
clusters as old as the majority of the Galactic GCs, our method do not
distinguish them accurately. We emphasize that our results show that even with
multiband photometry spanning NUV to $K_s$, our age constraints from SED
fitting are distressingly uncertain, which has implications for age derivations
in extragalactic globular cluster systems.
