  We measure the age-velocity relationship from the lag between ionized gas and
stellar tangential speeds in ~500 nearby disk galaxies from MaNGA in SDSS-IV.
Selected galaxies are kinematically axisymmetric. Velocity lags are asymmetric
drift, seen in the Milky Way's (MW) solar neighborhood and other Local Group
galaxies; their amplitude correlates with stellar population age. The trend is
qualitatively consistent in rate (d(sigma)/dt) with a simple power-law model
where sigma is proportional to t^b that explains the dynamical phase-space
stratification in the solar neighborhood. The model is generalized based on
disk dynamical times to other radii and other galaxies. We find in-plane radial
stratification parameters sigma_(0,r} (dispersion of the youngest populations)
in the range of 10-40 km/s and 0.2<b_r<0.5 for MaNGA galaxies. Overall b_r
increases with galaxy mass, decreases with radius for galaxies above 10.4 dex
(M_solar) in stellar mass, but is ~constant with radius at lower mass. The
measurement scatter indicates the stratification model is too simple to capture
the complexity seen in the data, unsurprising given the many possible
astrophysical processes that may lead to stellar population dynamical
stratification. Nonetheless, the data show dynamical stratification is broadly
present in the galaxy population, with systematic trends in mass and density.
The amplitude of the asymmetric drift signal is larger for the MaNGA sample
than the MW, and better represented in the mean by what is observed in the
disks of M31 and M33. Either typical disks have higher surface-density or, more
likely, are dynamically hotter (hence thicker) than the MW.
