  Using Galactic dynamics we have determined the age of the low mass post-T
Tauri stars TW Hya Association (TWA). To do so we applied the method of Ortega
et al.(2002, 2004) to five stars of the association with Hipparcos measured
distances (TWA 1, TWA 4,TWA 9,TWA 11,TWA 19).The method is based on the
calculation of the past 3D orbits of the stars. Of these stars only TWA 9
presents a quite different orbit so that it does not appear to be a dynamical
member of TWA. The four remaining stars have a first maximum orbits'
confinement at the age of -8.3 $\pm{0.8}$ Myr which is considered the dynamical
age of TWA. This confinement fixes the probable 3D forming region of TWA with a
mean radius of 14.5 pc. This region is related to the older subgroups of the
Sco-Cen OB association, Lower Centaurus Crux (LCC) and Upper Centaurus Lupus
(UCL), both with a mean age of about 18 Myr. This dynamical age of TWA and that
of the $\beta$ Pic Moving Group (BPMG) with an age of 11 Myr, also discussed
here, introduce a more precise temporal scale for studies of disks evolution
and planetary formation around some stars of these associations. Using the
retraced orbit of the runaway star HIP 82868 we examine the possibility that
the formation of TWA was triggered by a supernova (SN) explosion. It is shown
that for the four considered TWA stars, the expansion in volume is a factor of
five since their origin to the present state. This is mainly due to the
presently more distant star TWA 19.
