We performed a detailed analysis of the use of [C/N] measured in red giant branch stars between the completion of the first dredge up and the red giant branch bump ([C/N] FDU) as age indicator. [C/N] FDU cannot give accurate ages for individual stars, but may provide a general chronology for the formation of composite populations and add constraints to analyses of red giants from surface gravity-effective temperature diagrams. We provide a theoretical calibration of [C/N] FDU in terms of total metallicity [M/H] and age, for ages greater than ~1 Gyr, which we tested against variations in the initial heavy element distribution (scaled-solar vs. α-enhanced), efficiency of overshooting from MS convective cores and from the convective envelopes, variations in the initial He abundance and in the mixing length parameter. Our calibration is compared with a small sample of available measurements of [C/N] FDU in star clusters and halo field stars, which at least qualitatively confirm the overall trend of the predicted [C/N] FDU with age and [M/H]. The use of [C/N] FDU-[M/H]-age relations obtained from independent sets of stellar evolution calculations cause age differences (for a given [C/N] FDU and [M/H] pair) up to ~2 Gyr. More accurate spectroscopic measurements of [C/N] FDU in star clusters with well-established ages and metallicities are required to better test theoretical calibrations of this age indicator.