The colour-magnitude diagram (CMD) of NGC1851 presents two subgiant branches (SGBs), probably due to the presence of two populations differing in total C+N+O (carbon+nitrogen+oxygen) content. We test the idea that a difference in total C+N+O may simulate an age difference when comparing the CMD of clusters to derive relative ages. We compare NGC1851 with NGC6121 (M4), a cluster of very similar [Fe/H]. We find that, with a suitable shift of the CMDs that brings the two red horizontal branches at the same magnitude level, the unevolved main sequence and red giant branch match, but the SGB of NGC6121 and its red giant branch `bump' are fainter than in NGC1851. In particular, the SGB of NGC6121 is even slightly fainter than the faint SGB in NGC1851. Both these features can be explained if the total C+N+O in NGC6121 is larger than that in NGC1851, even if the two clusters are coeval. We conclude by warning that different initial C+N+O abundances between two clusters, otherwise similar in metallicity and age, may lead to differences in the turnoff morphology that can be easily attributed to an age difference. Based in part on observations made with the European Southern Observatory (ESO) telescopes obtained from the ESO/ST-ECF Science Archive Facility. This paper makes use of data obtained from the Isaac Newton Group Archive which is maintained as part of the CASU Astronomical Data Centre at the Institute of Astronomy, Cambridge. ‡ E-mail: dantona [at] oa-roma.inaf.it (dantona[at]oa-roma[dot]inaf[dot]it) (FD); peter.stetson [at] nrc-cnrc.gc.ca (peter[dot]stetson[at]nrc-cnrc[dot]gc[dot]ca) (PBS); ventura [at] oa-roma.inaf.it (ventura[at]oa-roma[dot]inaf[dot]it) (PV); antonino.milone [at] unipd.it (antonino[dot]milone[at]unipd[dot]it) (APM); giampaolo.piotto [at] unipd.it (giampaolo[dot]piotto[at]unipd[dot]it) (GP); vittoria.caloi [at] iasf-roma.inaf.it (vittoria[dot]caloi[at]iasf-roma[dot]inaf[dot]it) (VC)