Mapping the stellar age of the Milky Way bulge with the VVV. I. The
  method
  Recent observational programmes are providing a global view of the Milky Way
bulge that serves as template for detailed comparison with models and
extragalactic bulges. A number of surveys (i.e. VVV, GIBS, GES, ARGOS, BRAVA,
APOGEE) are producing comprehensive and detailed extinction, metallicity,
kinematics and stellar density maps of the Galactic bulge with unprecedented
accuracy. However, the still missing key ingredient is the distribution of
stellar ages across the bulge. To overcome this limitation, we aim to age-date
the stellar population in several bulge fields with the ultimate goal of
deriving an age map of the Bulge. This paper presents the methodology and the
first results obtained for a field along the Bulge minor axis, at $b=-6^\circ$.
We use a new PSF-fitting photometry of the VISTA Variables in the V\'{i}a
L\'{a}ctea (VVV) survey data to construct deep color-magnitude diagrams of the
bulge stellar population down to $\sim$ 2 mag below the Main Sequence turnoff.
We find the bulk of the bulge stellar population in the observed field along
the minor axis to be at least older than $\sim$ 7.5 Gyr. In particular, when
the metallicity distribution function spectroscopically derived by GIBS is
used, the best fit to the data is obtained with a combination of synthetic
populations with ages in between $\sim$ 7.5 Gyr and 11 Gyr. However, the
fraction of stars younger than $\sim$ 10 Gyr strongly depends upon the number
of Blue Straggler Stars present in the bulge. Simulations show that the
observed color-magnitude diagram of the bulge in the field along the minor axis
is incompatible with the presence of a conspicuous population of
intermediate-age/young (i.e. $\lesssim 5$ Gyr) stars.
