  Recent works have shown how the [C/N] ratio in stars after the first
dredge-up (FDU) can be used as an age estimator in virtue of its dependence on
stellar mass. For this purpose, precise predictions of the surface chemical
composition before and after the mixing takes place in the convective envelope
of subgiant stars are necessary. Stellar evolution models can provide us with
such predictions, although a comparision with objects of known age is needed
for calibration. Open clusters are excellent test cases, as they represent a
single stellar population for which the age can be derived through, e.g.,
isochrone fitting. In this study, we present a detailed analysis of stars
belonging to the well-known open cluster M67 observed by the APOGEE survey in
the twelfth data release of the Sloan Digital Sky Survey and whose chemical
properties were derived with the ASPCAP pipeline. We find that the [C/N]
abundance of subgiant branch stars is overestimated by $\sim0.2$ dex due to an
offset in the determination of the [N/Fe] abundance. Stars on the red giant
branch and red clump are shown not to be affected by this offset. We derive
$\text{[C/N]}_{FDU}=-0.46\pm 0.03$ dex, which poses a strong constraint on
calibrations of $\text{[C/N]}_{FDU}$ as age indicator. We also do not find any
clear signature of additional chemical mixing processes that set in after the
red giant branch bump. The results obtained for M67 indicate the importance of
conducting high-resolution spectroscopic studies of open clusters of different
ages in order to establish an accurate age-dating method for field stars.
