  The associations and moving groups of young stars are excellent laboratories
for investigating stellar formation in the solar neighborhood. Previous results
have confirmed that a non-negligible fraction of old main-sequence stars is
present in the lists of possible members of young stellar kinematic groups. A
detailed study of the properties of these samples is needed to separate the
young stars from old main-sequence stars with similar space motion, and
identify the origin of these structures. We used stars possible members of the
young (~ 10 - 650 Myr) moving groups from the literature. To determine the age
of the stars, we used several suitable age indicators for young main sequence
stars, i.e., X-ray fluxes and other photometric data. We also used
spectroscopic data, in particular the equivalent width of the lithium line Li I
and Halpha, to constrain the range of ages of the stars. By combining
photometric and spectroscopic data, we were able to separate the young stars
(10 - 650 Myr) from the old (> 1 Gyr) field ones. We found, in particular, that
the Local Association is contaminated by old field stars at the level of ~30%.
This value must be considered as the contamination for our particular sample,
and not of the entire Local Association. For other young moving groups, it is
more difficult to estimate the fraction of old stars among possible members.
However, the level of X-ray emission can, at least, help to separate two age
populations: stars with <200 Myr and stars older than this. Our results are
consistent with a scenario in which the moving groups contain both groups of
young stars formed in a recent star-formation episode and old field stars with
similar space motion. Only by combining X-ray and optical spectroscopic data is
it possible to distinguish between these two age populations.
