  Mid-infrared observations (3.6 - 24 microns) of normal giant elliptical
galaxies with the Spitzer space telescope are consistent with pure populations
of very old stars with no evidence of younger stars. Most of the stars in giant
elliptical galaxies are old but the mean stellar age determined from Balmer
absorption in optical spectra can appear much younger due to a small admixture
of younger stars. The mean stellar age can also be determined from the spectral
energy distribution in the mid-infrared which decreases with time relative to
the optical emission and shifts to shorter wavelengths. The observed flux
ratios F_8um/F_3.6um and F_24um/F_3.6um for elliptical galaxies with the oldest
Balmer line ages are lower than predicted by recent models of single stellar
populations. For ellipticals with the youngest Balmer line ages in our sample,
3-5 Gyrs, the flux ratios F_24um/F_3.6um are identical to those of the oldest
stars. When theoretical mid-IR spectra of old (12 Gyr) and young stellar
populations are combined, errors in the F_24um/F_3.6um observations are
formally inconsistent with a mass fraction of young stars that exceeds ~1%.
This is less than the fraction of young stars expected in discussions of recent
surveys of elliptical galaxies at higher redshifts. However, this inconsistancy
between Balmer line ages and those inferred from mid-IR observations must be
regarded as provisional until more accurate observations and theoretical
spectra become available. Finally, there is no evidence to date that central
disks or patches of dust commonly visible in optical images of elliptical
galaxies contribute sensibly to the mid-IR spectrum.
