rest-frame composite SED from the medium-band measurements of 46 reddest galaxies over the redshift range $0.9<z<1.4$ (also shown in Figure \[SEDs\]). If this model is then smoothed with a Gaussian with a width equal to the resolution of the medium-band filters, 0.15$\mu$m/(1+$z$), we see that the smoothed model at medium-band resolution (thick black line) is smoother than the model at original resolution, but still follows the shape of the Balmer/4000$\mathrm{\AA}$-break region closely. Note that we mark the location of the 4000$\mathrm{\AA}$-break with an arrow. We then smooth the models with a Gaussian that has a width equal to the typical resolution of broad-band filters, 0.4$\mu$m/(1+$z$) (dashed line). The Balmer/4000$\mathrm{\AA}$-break has become washed out due to the poor resolution. []{data-label="resolution"}](fig5_sm.eps)

Stellar populations with ages of $\sim$0.8 Gyr will have strong Balmer breaks at 3646$\mathrm{\AA}$, whereas older populations will have pronounced 4000Å breaks. Although the differences will be subtle, the SEDs of galaxies with ages of $\sim$0.8-1 Gyr should be distinct from older stellar populations. This has not been observed previously at high $z$ because the $J$, $H$, and $K$ broad-band filters smooth over these spectral features. Figure \[resolution\] demonstrates the importance of the spectral resolution by comparing the composite SED of 