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In the era of large high-resolution spectroscopic surveys, high-qualityspectra can contribute to our understanding of the Galactic chemical evolution,providing chemical abundances belonging to the different nucleosynthesischannels, and also providing constraints to stellar age. Some abundance ratioshave been proven to be excellent indicators of stellar ages. We aim atproviding an empirical relationship between stellar ages and [C/N] using, ascalibrators, open star clusters observed by both the Gaia-ESO and APOGEEsurveys. We use stellar parameters and abundances from the Gaia-ESO and APOGEEof the Galactic field and open cluster stars. Ages of star clusters areretrieved from the literature sources and validated using a common set ofisochrones. We use the same isochrones to determine, for each age andmetallicity, the surface gravity at which the first dredge-up and red giantbranch bump occur. We study the effect of extra-mixing processes in our sampleof giant stars, and we derive the mean [C/N] in evolved stars, including onlystars without evidence of extra-mixing. Combining the Gaia-ESO and APOGEEsamples of open clusters, we derive a linear relationship between [C/N] andlogarithmic cluster ages. We apply our relationship to selected giant fieldstars in both Gaia-ESO and APOGEE. We find an age separation between thin andthick disc stars and age trends within their populations, with an increasingage towards lower metallicity populations. With such empirical relationship, weare able to provide an age estimate for giant stars in which C and N abundancesare measured. Isochrone fitting is less sensitive for giant than dwarf stars atthe turn off. The present method can be thus considered as an additional toolto give an independent estimate of the age of giant stars, with uncertaintiesin their ages comparable to those obtained using isochrone fitting for dwarfstars.

The Gaia-ESO survey: Calibrating a relationship between Age and the [C/N] abundance ratio with open clusters