  We investigated the absolute age of the Galactic globular cluster M71 (NGC
6838) by using optical ground-based images (u',g',r',i',z') collected with the
MegaCam camera at the Canada-France-Hawaii-Telescope (CFHT). We performed a
robust selection of field and cluster stars by applying a new method based on
the 3D (r',u'-g',g'-r') Color-Color-Magnitude-Diagram. The comparison between
the Color-Magnitude-Diagram of the candidate cluster stars and a new set of
isochrones, at the locus of the Main Sequence Turn Off (MSTO), suggests an
absolute age of 12+/-2 Gyr. The absolute age was also estimated using the
difference in magnitude between the MSTO and the so-called main sequence knee,
a well defined bending occurring in the lower main sequence. This feature was
originally detected in the near-infrared (NIR) bands and explained as a
consequence of an opacity mechanism (collisionally induced absorption of
molecular hydrogen) in the atmosphere of cool low-mass stars (Bono et al.
2010). The same feature was also detected in the r',u'-g' and in the r',g'-r'
CMD, thus supporting previous theoretical predictions by Borysow et al. (1997).
The key advantage in using the Delta^(Knee)_(TO) as an age diagnostic is that
it is independent of uncertainties affecting the distance, the reddening and
the photometric zero-point. We found an absolute age of 12+/-1 Gyr that agrees,
within the errors, with similar age estimates, but the uncertainty is on
average a factor of two smaller. We also found that the Delta^(Knee)_(TO) is
more sensitive to the metallicity than the MSTO, but the dependence becomes
vanishing using the difference in color between the MSK and the MSTO.
