  We present low-resolution near-infrared stacked spectra from the 3D-HST
survey up to $z=2.0$ and fit them with commonly used stellar population
synthesis models: BC03 (Bruzual & Charlot, 2003), FSPS10 (Flexible Stellar
Population Synthesis, Conroy & Gunn 2010), and FSPS-C3K (Conroy, Kurucz,
Cargile, Castelli, in prep). The accuracy of the grism redshifts allows the
unambiguous detection of many emission and absorption features, and thus a
first systematic exploration of the rest-frame optical spectra of galaxies up
to $z=2$. We select massive galaxies ($\rm log(M_{*} / M_{\odot}) > 10.8$), we
divide them into quiescent and star-forming via a rest-frame color-color
technique, and we median-stack the samples in 3 redshift bins between $z=0.5$
and $z=2.0$. We find that stellar population models fit the observations well
at wavelengths below $\rm 6500 \AA$ rest-frame, but show systematic residuals
at redder wavelengths. The FSPS-C3K model generally provides the best fits
(evaluated with a $\chi^2_{red}$ statistics) for quiescent galaxies, while BC03
performs the best for star-forming galaxies. The stellar ages of quiescent
galaxies implied by the models, assuming solar metallicity, vary from 4 Gyr at
$z \sim 0.75$ to 1.5 Gyr at $z \sim 1.75$, with an uncertainty of a factor of 2
caused by the unknown metallicity. On average the stellar ages are half the age
of the Universe at these redshifts. We show that the inferred evolution of ages
of quiescent galaxies is in agreement with fundamental plane measurements,
assuming an 8 Gyr age for local galaxies. For star-forming galaxies the
inferred ages depend strongly on the stellar population model and the shape of
the assumed star-formation history.
