  Recent spectroscopic observations in the Milky Way suggest that the
chemically defined thick disk (stars with high [alpha/Fe] ratios and thus old)
has a significantly smaller scale-length than the thin disk. This is in
apparent contradiction with observations of external edge-on galaxies, where
the thin and thick components have comparable scale-lengths. Moreover, while
observed disks do not flare (scale-height does not increase with radius),
numerical simulations suggest that disk flaring is unavoidable, resulting from
both environmental effects and secular evolution. Here we address these
problems by studying two different suites of simulated galactic disks formed in
the cosmological context. We show that the scale-heights of coeval populations
always increase with radius. However, the total population can be decomposed
morphologically into thin and thick disks, which do not flare. We relate this
to the disk inside-out formation, where younger populations have increasingly
larger scale-lengths and flare at progressively larger radii. In this new
picture, thick disks are composed of the imbedded flares of mono-age stellar
populations. Assuming that disks form inside out, we predict that
morphologically defined thick disks must show a decrease in age (or [alpha/Fe]
ratios) with radius and that coeval populations should always flare. This also
explains the observed inversion in the metallicity and [alpha/Fe] gradients for
stars away from the disk midplane in the Milky Way. The results of this work
are directly linked to, and can be seen as evidence of, inside-out disk growth.
