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The associations and moving groups of young stars are excellent laboratoriesfor investigating stellar formation in the solar neighborhood. Previous resultshave confirmed that a non-negligible fraction of old main-sequence stars ispresent in the lists of possible members of young stellar kinematic groups. Adetailed study of the properties of these samples is needed to separate theyoung stars from old main-sequence stars with similar space motion, andidentify the origin of these structures. We used stars possible members of theyoung (~ 10 - 650 Myr) moving groups from the literature. To determine the ageof the stars, we used several suitable age indicators for young main sequencestars, i.e., X-ray fluxes and other photometric data. We also usedspectroscopic data, in particular the equivalent width of the lithium line Li Iand Halpha, to constrain the range of ages of the stars. By combiningphotometric 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, thatthe 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 ismore 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 agepopulations: stars with <200 Myr and stars older than this. Our results areconsistent with a scenario in which the moving groups contain both groups ofyoung stars formed in a recent star-formation episode and old field stars withsimilar space motion. Only by combining X-ray and optical spectroscopic data isit possible to distinguish between these two age populations.

Quantifying the contamination by old main-sequence stars in young moving groups: the case of the Local Association