We investigate the Spitzer/IRAC properties of 36 z~7 z-dropout galaxies and 3 z~8 Y-dropout galaxies derived from deep/wide-area WFC3/IR data of the Early Release Science, the ultradeep HUDF09, and wide-area NICMOS data. We fit stellar population synthesis models to the SEDs to derive mean redshifts, stellar masses, and ages. The z~7 galaxies are best characterized by high ages (>300 Myr) and high M/L. The main trend with decreasing luminosity is that of bluing of the far-UV slope from beta ~ -2.0 to beta ~ -3.0. This can be explained by decreasing metallicity, except for the lowest luminosity galaxies (0.1 L^*_{z=3}) where low metallicity with smooth SFHs alone fail to match the blue far-UV and moderately red H-[3.6] color. This may require episodic SFHs with short periods of activity and quiescence ("on-off" cycles) or contribution from emission lines. The stellar mass of our sample of z~7 star forming galaxies correlates with SFR according to log M*= 8.70 (+-0.09) + 1.06 (+-0.10) log SFR. The small scatter ~0.25 dex suggest that the galaxies have similar SFH, on average consistent with CSF since z>10. No galaxies are found with SFRs much higher or lower than the past averaged SFR; strongly rising SFR \propto t^alpha (alpha>1) or exponentially declining tau<t_Hubble SFHs are disfavored. We report the first robust IRAC detection of Y_098-dropout galaxies at z~8. The average rest-frame U - V ~ 0.3 (AB) of the 3 galaxies are similar to faint z~7 galaxies, implying similar M/L and age. The stellar mass density to M_{UV,AB}<-18 is rho*(z=8)=1.8^{+0.7}_{-1.0} x 10^6 M_sun Mpc^{-3}, following log rho^*(z)=11.99-6 log(1+z) [M_sun Mpc^{-3}] over 3<z<8.