We use deep textit{Hubble Space Telescope} spectroscopy to constrain the metallicities and (editone{light-weighted}) ages of massive ($log M_ast/M_odotgtrsim10$) galaxies selected to have quiescent stellar populations at $1.0<z<1.8$. The data include 12--orbit depth coverage with the WFC3/G102 grism covering $sim$ $8,000<lambda<11,500$~AA, at a spectral resolution of $Rsim 210$ taken as part of the CANDELS Lyman-$alpha$ Emission at Reionization (CLEAR) survey. At $1.0<z<1.8$, the spectra cover important stellar population features in the rest-frame optical. We simulate a suite of stellar population models at the grism resolution, fit these to the data for each galaxy, and derive posterior likelihood distributions for metallicity and age. We stack the posteriors for subgroups of galaxies in different redshift ranges that include different combinations of stellar absorption features. Our results give editone{light-weighted ages of $t_{z sim 1.1}= 3.2pm 0.7$~Gyr, $t_{z sim 1.2}= 2.2pm 0.6$~Gyr, $t_{zsim1.3}= 3.1pm 0.6$~Gyr, and $t_{zsim1.6}= 2.0 pm 0.6$~Gyr, editone{for galaxies at $zsim 1.1$, 1.2, 1.3, and 1.6. This} implies that most of the massive quiescent galaxies at $1<z<1.8$ had formed $>68$% of their stellar mass by a redshift of $z>2$}. The posteriors give metallicities of editone{$Z_{zsim1.1}=1.16 pm 0.29$~$Z_odot$, $Z_{zsim1.2}=1.05 pm 0.34$~$Z_odot$, $Z_{zsim1.3}=1.00 pm 0.31$~$Z_odot$, and $Z_{zsim1.6}=0.95 pm 0.39$~$Z_odot$}. This is evidence that massive galaxies had enriched rapidly to approximately Solar metallicities as early as $zsim3$.
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