  X-rays and extreme ultraviolet radiation impacting on a gas produce a variety
of effects that, depending on the electron content, may provide a significant
heating of the illuminated region. In a planetary atmosphere of solar
composition, stellar high energy radiation may heat the gas to very high
temperatures, that may have consequences on the stability of planetary
atmospheres, in particular for close-in planets.
  We investigate the variations with stellar age in the occurring frequency of
gas giant planets orbiting G and M stars, taking into account that the high
energy luminosity of a low mass star evolves in time, both in intensity and
hardness.
  Using the energy-limited escape approach we investigate the effects induced
by the atmospheric mass loss on giant exoplanet distribution that is initially
flat, at several distances from the parent star. We follow the dynamical
evolution of the planet atmosphere, tracking the departures from the initial
profile due to the atmospheric escape, until it reaches the final mass-radius
configuration.
  We find that a significant fraction of low mass Jupiter-like planets orbiting
with periods lower than ~3.5 days either vaporize during the first billion
years, or lose a relevant part of their atmospheres. The planetary initial mass
profile is significantly distorted; in particular, the frequency of occurrence
of gas giants, less massive than 2 MJ, around young star can be considerably
greater than the one around older stellar counterparts.
