  In this work we study the stellar mass -- metallicity relation (MZR) of an
extended sample of star-forming galaxies in the local Universe and its possible
dependence with the star formation rate (SFR). A sample of $\sim$195000 Sloan
Digital Sky Survey (SDSS) star-forming galaxies has been selected up to z=0.22
with the aim of analysing the behaviour of the relation of MZR with respect to
SFR and taking into account the age of their stellar populations. For this
sample we have obtained, for the first time, aperture corrected oxygen and
nitrogen-to-oxygen abundances (O/H and N/O, respectively) and SFR using the
empirical prescriptions from the Calar Alto Legacy Integral Field Area (CALIFA)
survey. To perform this study we make use also of the stellar mass of the
galaxies and the parameter Dn(4000) as a proxy of the age of the stellar
population. We derive a robust MZR locus, which is found to be fully consistent
with the "anchoring" points of a selected set of well studied nearby galaxies
with a direct derivation of the chemical abundance. A complex relation between
MZR and SFR across the whole range of galaxy mass and metallicity has been
observed, where the slope changes seen in the O/H -- SFR plane present a
pattern which seems to be tuned to the galaxies' stellar age, and therefore,
stellar age has to be taken into account in the stellar mass -- metallicity --
SFR relation. In order to provide an answer to the question of whether or not
the MZR depends on the SFR it is essential to take into account the age of the
stellar populations of galaxies. A strong dependence between the MZR and SFR is
observed mainly for star-forming galaxies with strong SFR values and low
Dn(4000). The youngest galaxies of our SDSS sample show the highest SFR
measured for their stellar mass.
