  The kinematics of about 40 single stars belonging to the $\beta$ Pictoris
moving group is studied. The age of the $\beta$ Pictoris moving group is
estimated from these stars with ground-based line-of-sight velocity
determinations by two methods. Both estimates are kinematic. In the first
method we considered the traceback trajectories of the stars, giving an
estimate of $t=13.2\pm1.4$ Myr. In the second method, by analyzing the
instantaneous velocities of the stars, we show that there is an expansion of
the stellar system occurring in the Galactic $xy$ plane. Based on this effect,
we find the time interval elapsed from the beginning of the expansion of the
$\beta$ Pictoris moving group to the present day, $t=20\pm2$ Myr.
