We investigated the Galactic globular cluster M71 (NGC6838, [Fe/H]=-0.78) by using optical (u', g', r', i', z') ground-based images, collected with the MegaCam camera at the Canadian-Franch-Hawaii-Telescope (CFHT). We provided a robust selection of the field and cluster candidate stars by using a new approach based on a 3D Color-Color-Magnitude-Diagram (CCMD). In order to investigate the cluster age, we compared the observed CMDs of the candidate cluster stars with appropriate isochrones. We found an age of 12±2.0 Gyr, in good agreement with literature values. Beside the classical isochrone comparison, we also estimated the age by using the 'knee method', which is independent by the uncertainties affecting the cluster distance and reddening. By applying the latter method to the r',g'-r' and r',u'-g' CMDs, we found a mean age of 12±1.0 Gyr. We found that the different methods suggest the same age, but the knee-method allows us to decrease the uncertainty by a factor of two. Our results confirm that M71 is an old metal-rich Galactic globular cluster.