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Deep rest-optical observations are required to accurately constrain thestellar populations of $z\sim8$ galaxies. Due to significant limitations in theavailability of such data for statistically complete samples, observationalresults have been limited to modest numbers of bright or lensed sources. Torevolutionize the present characterization of $z\sim8$ galaxies, we exploit theultradeep ($\sim27$ mag, $3\sigma$) Spitzer/IRAC $3.6\mu$m and $4.5\mu$m data,probing the rest-frame optical at $z\sim8$, over $\sim200$ arcmin$^2$ of theGOODS fields from the recently completed GOODS Re-ionization Era wide-AreaTreasury from Spitzer (GREATS) program, combined with observations in theCANDELS UDS and COSMOS fields. We stacked $\gtrsim100$ $z\sim8$ Lyman-Breakgalaxies in four bins of UV luminosity ($M_\mathrm{UV}\sim -20.7$ to $-18.4$)and study their $H_\mathrm{160}-[3.6]$ and $[3.6]-[4.5]$ colors. We find youngages ($\lesssim100$ Myr) for the three faintest stacks, inferred from theirblue $H_\mathrm{160}-[3.6]\sim 0$ mag colors, consistent with a negative Balmerbreak. Meanwhile, the redder $H_\mathrm{160}-[3.6]$ color seen in the brighteststack is suggestive of slightly older ages. We explored the existence of acorrelation between the UV luminosity and age, and find either no trend orfainter galaxies being younger. The stacked SEDs also exhibit very red$[3.6]-[4.5]\sim0.5$ mag colors, indicative of intense [OIII]+H$\beta$ nebularemission and SFR. The correspondingly high specific star-formation rates,sSFR$\gtrsim10$Gyr$^{-1}$, are consistent with recent determinations at similarredshifts and higher luminosities, and support the co-evolution between thesSFR and the specific halo mass accretion rate.

Blue Rest-Frame UV-Optical Colors in z~8 Galaxies from GREATS: Very Young Stellar Populations at ~650 Myr of Cosmic Time