  The composition and structure of the upper atmospheres of Extrasolar Giant
Planets (EGPs) are affected by the high-energy spectrum of their host stars
from soft X-rays to EUV. This emission depends on the activity level of the
star, which is primarily determined by its age. We focus upon EGPs orbiting K-
and M-dwarf stars of different ages. XUV spectra for these stars are
constructed using a coronal model. These spectra are used to drive both a
thermospheric model and an ionospheric model, providing densities of neutral
and ion species. Ionisation is included through photo-ionisation and
electron-impact processes. We find that EGP ionospheres at all orbital
distances considered and around all stars selected are dominated by the
long-lived H$^+$ ion. In addition, planets with upper atmospheres where H$_2$
is not substantially dissociated have a layer in which H$_3^+$ is the major ion
at the base of the ionosphere. For fast-rotating planets, densities of
short-lived H$_3^+$ undergo significant diurnal variations, with the maximum
value being driven by the stellar X-ray flux. In contrast, densities of
longer-lived H$^+$ show very little day/night variability and the magnitude is
driven by the level of stellar EUV flux. The H$_3^+$ peak in EGPs with upper
atmospheres where H$_2$ is dissociated under strong stellar illumination is
pushed to altitudes below the homopause, where this ion is likely to be
destroyed through reactions with heavy species. The inclusion of secondary
ionisation processes produces significantly enhanced ion and electron densities
at altitudes below the main EUV ionisation peak, as compared to models that do
not include electron-impact ionisation. We estimate infrared emissions from
H$_3^+$, and while, in an H/H$_2$/He atmosphere, these are larger from planets
orbiting close to more active stars, they still appear too low to be detected
with current observatories.
