  Numerical simulations of galaxy formation within the cold dark matter (CDM)
hierarchical clustering framework are unable to produce large disk galaxies
without invoking some form of feedback to suppress gas cooling and collapse to
a redshift of unity or below. An important observational consequence of
delaying the epoch of disk formation until relatively recent times is that the
stellar populations in the extended disk should of be predominantly
young-to-intermediate age. We use a deep HST/WFPC2 archival pointing to
investigate the mean age and metallicity of the stellar population in a
disk-dominated field at 30 kpc along the major axis of M31. Our analysis of the
color-magnitude-diagram reveals the dominant population to have significant
mean age (>~8 Gyr) and a moderately-high mean metallicity ([Fe/H]~-0.7);
tentative evidence is also presented for a trace population of ancient (>10
Gyr) metal-poor stars. These characteristics are unexpected in CDM models and
we discuss the possible implications of this result, as well as alternative
interpretations.

Astrophysics