  We present a detailed stellar population analysis of 11 bright ($H<26.6$)
galaxies at $z=9-11$ (three spectroscopically confirmed) to constrain the
chemical enrichment and growth of stellar mass of early galaxies. We use the
flexible Bayesian spectral energy distribution (SED) fitting code Prospector
with a range of star-formation histories (SFHs), a flexible dust attenuation
law and a self-consistent modeling of emission lines. This approach allows us
to assess how different priors affect our results, and how well we can break
degeneracies between dust attenuation, stellar ages, metallicity and emission
lines using data which probe only the rest-frame ultraviolet to optical
wavelengths. We measure a median observed ultraviolet spectral slope
$\beta=-1.87_{-0.43}^{+0.35}$ for relatively massive star-forming galaxies
($9<\log(M_{\star}/M_{\odot})<10$), consistent with no change from $z=4$ to
$z=9-10$ at these stellar masses, implying rapid enrichment. Our SED-fitting
results are consistent with a star-forming main sequence with sub-linear slope
($0.7\pm0.2$) and specific star-formation rates of $3-10~\mathrm{Gyr}^{-1}$.
However, the stellar ages and SFHs are less well constrained. Using different
SFH priors, we cannot distinguish between median mass-weighted ages of
$\sim50-150$ Myr, which corresponds to 50\% formation redshifts of
$z_{50}\sim10-12$ at $z\sim9$ and is of the order of the dynamical timescales
of these systems. Importantly, the models with different SFH priors are able to
fit the data equally well. We conclude that the current observational data
cannot tightly constrain the mass-buildup timescales of these $z=9-11$
galaxies, with our results consistent with SFHs implying both a shallow and
steep increase of the cosmic SFR density with time at $z>10$.
