  (Abridged) Age derivation techniques for unresolved stellar populations at
high redshifts are explored using the NUV spectrum of LBDS~53W091 and
LBDS~53W069. The photometry and morphology of these galaxies suggest they are
early-type systems,a feature that makes them ideal test beds for the analysis
of their ages and metallicities. In the analysis that is based on simple
stellar population models,we find a significant degeneracy between the derived
ages and metallicities both in optical+NIR photometric and NUV spectroscopic
analyses. This degeneracy is not so strong for LBDS~53W069. However even in
this case the stellar age cannot be constrained better than to a range roughly
encompassing one third of the age of the Universe at the observed redshift. We
have explored several independent population synthesis models and consistently
found similar results. Broadband photometry straddling the rest-frame 4000A
break is also subject to a strong age-metallicity degeneracy. The use of
realistic chemical enrichment assumptions significantly helps in disentangling
the degeneracy. Based on this method, we derive the average stellar age for
both galaxies around 3.6-3.8 Gyr with better constraints on the youngest
possible ages. From the observational point of view, the most efficient (and
feasible) way to set limits on unresolved stellar populations comprises a
combination of Balmer absorption lines along with either low SNR rest frame NUV
spectroscopy or accurate optical and NIR photometry.
