Context : 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 .

Aims : Our intention is to characterize members of the young moving groups , determine their age distribution , and quantify the contamination by old main-sequence stars , in particular , for the Local Association .

Methods : We used stars possible members of the young ( \sim 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 from the Rosat All-sky Survey database , photometric data from the Tycho-2 , Hipparcos , and 2MASS database .
We also used spectroscopic data , in particular the equivalent width of the lithium line Li i \lambda 6707.8 à  and H _ { \alpha } , to constrain the range of ages of the stars .

Results : By combining photometric and spectroscopic data , we were able to separate the young stars ( 10 - 650 Â 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 \sim 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 Â Myr and stars older than this .

Conclusions : Among the candidate members of the classical moving groups , there is a non-negligible fraction of old field stars that should be taken into account when studying the stellar birthrate in the solar neighborhood .
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 .