  The orbit distribution of young stars in the Galactic disk is highly
structured, from well-defined clusters to streams of stars that may be widely
dispersed across the sky, but are compact in orbital action-angle space. The
age distribution of such groups can constrain the timescales over which
co-natal groups of stars disperse into the `field'. Gaia data have proven
powerful to identify such groups in action-angle space, but the resulting
member samples are often too small and have too narrow a CMD coverage to allow
robust age determinations. Here, we develop and illustrate a new approach that
can estimate robust stellar population ages for such groups of stars. This
first entails projecting the predetermined action-angle distribution into the
5D space of positions, parallaxes and proper motions, where much larger samples
of likely members can be identified over a much wider range of the CMD. It then
entails isochrone fitting that accounts for a) widely varying distances and
reddenings; b) outliers and binaries; c) sparsely populated main sequence
turn-offs, by incorporating the age information of the low-mass main sequence;
and d) the possible presence of an intrinsic age spread in the stellar
population. When we apply this approach to 92 nearby stellar groups identified
in 6D orbit space, we find that they are predominately young ($\lesssim 1$
Gyr), mono-age populations. Many groups are established (known) localized
clusters with possible tidal tails, others tend to be widely dispersed and
manifestly unbound. This new age-dating tool offers a stringent approach to
understanding on which orbits stars form in the solar neighborhood and how
quickly they disperse into the field.
