Evolution over time of the Milky Way's disc shape
  Context. Galactic structure studies can be used as a path to constrain the
scenario of formation and evolution of our Galaxy. The dependence with the age
of stellar population parameters would be linked with the history of star
formation and dynamical evolution. Aims. We aim to investigate the structures
of the outer Galaxy, such as the scale length, disc truncation, warp and flare
of the thin disc and study their dependence with age by using 2MASS data and a
population synthesis model (the so-called Besan\c{c}on Galaxy Model). Methods.
We have used a genetic algorithm to adjust the parameters on the observed
colour-magnitude diagrams at longitudes 80 deg <= l <= 280 deg for |b| <= 5.5
deg. We explored parameter degeneracies and uncertainties. Results. We identify
a clear dependence of the thin disc scale length, warp and flare shapes with
age. The scale length is found to vary between 3.8 kpc for the youngest to
about 2 kpc for the oldest. The warp shows a complex structure, clearly
asymmetrical with a node angle changing with age from approximately 165 deg for
old stars to 195 deg for young stars. The outer disc is also flaring with a
scale height that varies by a factor of two between the solar neighbourhood and
a Galactocentric distance of 12 kpc. Conclusions. We conclude that the thin
disc scale length is in good agreement with the inside-out formation scenario
and that the outer disc is not in dynamical equilibrium. The warp deformation
with time may provide some clues to its origin.
