We study the properties of dust in disks to constrain models of planet formation. We measure and analyze the spectral index for the dust continuum emission at millimeter wavelengths for a sample of 24 young disks in the Upper Sco star-forming region. We do this by combining data taken with the ALMA telescope at wavelengths of 2.87 mm and 0.88 mm. Since the age of this region is ∼ 5 - 10 Myr, these results can constrain the properties of small solids in disks at the end of their lifetime. We examine whether dust trapping, which is key to the formation of planetesimals, happens only in much younger disks or if it is efficient all the way towards the end of the disk life cycle. Our results indicate that dust traps are present also in the relatively old disks in our sample, indicating that protoplanetary disks have the potential to form planetesimals during their entire lifetime. Our analysis also quantifies the effects of scattering by dust of the disk emission, a mechanism that has been recently proposed as potentially important to determine the fluxes of protoplanetary disks even at sub-mm/mm wavelengths. From preliminary results, based on the state-of-the-art radiative transfer code RADMC-3d, we infer that scattering is not effective for optically thin disk models, but could potentially play a significant role for optically thick models.