Ivo Labbe (OCIW) Valentino Gonzalez, (USCS) Bouwens (UCSC/Leiden), Illingworth (UCSC). We study the Spitzer/IRAC mid-infrared (rest-frame optical) fluxes of new WFC3/IR-detected z~7 z- dropout galaxies and z~8 Y-dropout galaxies over the HUDF and CDF-S ERS. We construct average broadband SEDs using the stacked ACS, WFC3, and IRAC fluxes and fit stellar population synthesis models to derive mean redshifts, stellar masses, and ages. The most remarkable finding is that low-luminosity z~7 galaxies are moderately red and require high best-fit ages and appreciable M/L. One implication is that sub-L* galaxies contribute significantly to the stellar mass density and under favorable conditions may have provided enough photons for sustained reionization at 7 < z <11. We discuss other results including the relation between SFR and stellar mass at z=7 and the evolution of the stellar mass density to higher redshifts z>8.