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Using {\sl Gaia} Data Release 2 photometry, we report the detection ofextended main-sequence turnoff (eMSTO) regions in the color--magnitude diagrams(CMDs) of the $\sim 14$ Myr-old double clusters $h$ and $\chi$ Persei (NGC 869and NGC 884). We find that stars with masses below $\sim$1.3 $M_{\odot}$ inboth $h$ and $\chi$ Persei populate narrow main sequences (MSs), while moremassive stars define the eMSTO, closely mimicking observations of youngGalactic and Magellanic Cloud clusters (with ages older than $\sim$30 Myr).Previous studies based on clusters older than $\sim$30 Myr find that rapidlyrotating MS stars are redder than slow rotators of similar luminosity,suggesting that stellar rotation may be the main driver of the eMSTO. Bycombining photometry and projected rotational velocities from the literature ofstars in $h$ and $\chi$ Persei, we find no obvious relation between therotational velocities and colors of non-emission-line eMSTO stars, in contrastwith what is observed in older clusters. Similarly to what is observed inMagellanic Cloud clusters, most of the extremely rapidly rotating stars,identified by their strong H$\alpha$ emission lines, are located in the redpart of the eMSTOs. This indicates that stellar rotation plays a role in thecolor and magnitude distribution of MSTO stars. By comparing the observationswith simulated CMDs, we find that a simple population composed of coeval starsthat span a wide range of rotation rates is unable to reproduce the colorspread of the clusters' MSs. We suggest that variable stars, binaryinteractions, and stellar rotation affect the eMSTO morphology of these veryyoung clusters.

Extended main-sequence turnoffs in the double cluster $h$ and $\chi$ Persei: The complex role of stellar rotation