  Chromospheric activity has been calibrated and widely used as an age
indicator. However, it has been suggested that the viability of this age
indicator is, in the best case, limited to stars younger than about 1.5 Gyr. I
aim to define the age range for which chromospheric activity is a robust
astrophysical clock. I collected literature measurements of the S-index in
field stars, which is a measure of the strength of the H and K lines of the Ca
II and a proxy for chromospheric activity, and exploited the homogeneous
database of temperature and age determinations for field stars provided by the
Geneva-Copenhagen survey of the solar neighbourhood. Field data, inclusive data
previously used to calibrate chromospheric ages, confirm the result found using
open cluster data, i.e. there is no decay of chromospheric activity after about
2 Gyr. The only existing indication supporting the viability of chromospheric
ages older than 2 Gyr is the similarity of chromospheric activity levels in the
components of 35 dwarf binaries. However, even in the most optimistic scenario,
uncertainty in age determination for field stars and lack of sufficient data in
open clusters make any attempt of calibrating an age activity relationship for
old stars premature. The hypothesis that chromospheric activity follows the
Skumanich law, i.e. that it is proportional to the inverse square root of the
age, should be relaxed.
