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We analyze the present-day structure and assembly history of a highresolution hydrodynamic simulation of the formation of a Milky Way (MW)-likedisk galaxy, from the "Eris" simulation suite, dissecting it into cohorts ofstars formed at different epochs of cosmic history. At z=0, stars with t_form <2 Gyr mainly occupy the stellar spheroid, with the oldest (earliest forming)stars having more centrally concentrated profiles. The younger age cohortspopulate disks of progressively longer radial scale length and shorter verticalscale height. At a given radius, the vertical density profiles and velocitydispersions of stars vary smoothly as a function of age, and the superpositionof old, vertically-extended and young, vertically-compact cohorts gives rise toa double-exponential profile like that observed in the MW. Turning to formationhistory, we find that the trends of spatial structure and kinematics withstellar age are largely imprinted at birth, or immediately thereafter. Starsthat form during the active merger phase at z>3 are quickly scattered intorounded, kinematically hot configurations. The oldest disk cohorts form instructures that are radially compact and relatively thick, while subsequentcohorts form in progressively larger, thinner, colder configurations from gaswith increasing levels of rotational support. The disk thus forms "inside-out"in a radial sense and "upside-down" in a vertical sense. Secular heating andradial migration influence the final state of each age cohort, but the changesthey produce are small compared to the trends established at formation. Thepredicted correlations of stellar age with spatial and kinematic structure arein good qualitative agreement with the correlations observed for mono-abundancestellar populations in the MW.

Inside Out and Upside Down: Tracing the Assembly of a Simulated Disk Galaxy Using Mono-Age Stellar Populations