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With a complete, mass-selected sample of quiescent galaxies from the NEWFIRMMedium-Band Survey (NMBS), we study the stellar populations of the oldest andmost massive galaxies (>10^11 Msun) to high redshift. The sample includes 570quiescent galaxies selected based on their extinction-corrected U-V colors outto z=2.2, with accurate photometric redshifts, sigma_z/(1+z)~2%, and rest-framecolors, sigma_U-V~0.06 mag. We measure an increase in the intrinsic scatter ofthe rest-frame U-V colors of quiescent galaxies with redshift. This scatter incolor arises from the spread in ages of the quiescent galaxies, where we seeboth relatively quiescent red, old galaxies and quiescent blue, youngergalaxies towards higher redshift. The trends between color and age areconsistent with the observed composite rest-frame spectral energy distributions(SEDs) of these galaxies. The composite SEDs of the reddest and bluestquiescent galaxies are fundamentally different, with remarkably well-defined4000A- and Balmer-breaks, respectively. Some of the quiescent galaxies may beup to 4 times older than the average age- and up to the age of the universe, ifthe assumption of solar metallicity is correct. By matching the scatterpredicted by models that include growth of the red sequence by thetransformation of blue galaxies to the observed intrinsic scatter, the dataindicate that most early-type galaxies formed their stars at high redshift witha burst of star formation prior to migrating to the red sequence. The observedU-V color evolution with redshift is weaker than passive evolution predicts;possible mechanisms to slow the color evolution include increasing amounts ofdust in quiescent galaxies towards higher redshift, red mergers at z<1, and afrosting of relatively young stars from star formation at later times.

The Age Spread of Quiescent Galaxies with the NEWFIRM Medium-band Survey: Identification of the Oldest Galaxies out to z~2