  Recent studies based on line indices of nearby early-type galaxies suggest
that stars in the galaxies' inner part may be younger and more metal rich than
stars in the outer part. If confirmed, the finding has a profound implication
for the formation and evolution of early-type galaxies, since such age
gradients naturally arise if merging played an important role in their
formation/evolution. As an indpendent test for the existence of the age
gradient, we investigate the optical-NIR color gradients of six field
early-type galaxies at z \simeq 0.4 -- 1.0 where the age gradient is expected
to be detectable when combined with z=0 calibrations. By utilizing HST NICMOS
H-band imaging along with WFPC2 optical data, we obtain a broad wavelength
baseline, giving results that are sensitive to the expected age gradient. When
compared with simple model predictions, five of the objects show negligible age
gradients and are metallicity gradient dominated, while the remaining object
shows a color gradient consistent with the age+metallicity gradient picture.
Our results suggest that stars within early-type galaxies may be coeval at z
\sim 1, but sprinkling of young stars might have occured at z < 1.
