  It is well known that the magnetic activity of solar type stars decreases
with age, but it is widely debated in the literature whether there is a smooth
decline or if there is an early sharp drop until 1-2 Gyr followed by a
relatively inactive constant phase. We revisited the activity-age relation
using time-series observations of a large sample of solar twins whose precise
isochronal ages and other important physical parameters have been determined.
We measured the Ca II H and K activity indices using 9000 HARPS spectra of 82
solar twins. We measured an average solar activity of $S_{\rm MW}$ = 0.1712
$\pm$ 0.0017 during solar magnetic cycles 23$-$24 covered by HARPS observations
and we also inferred an average of $S_{\rm MW}$ = 0.1694 $\pm$ 0.0025 for
cycles 10$-$24, anchored on a S index vs. sunspot number correlation. Also, a
simple relation between the average and dispersion of the activity levels of
solar twins was found. This enabled us to predict the stellar variability
effects on the age-activity diagram and, consequently, estimate the
chromospheric age uncertainties due to the same phenomena. The age-activity
relation is still statistically significant up to ages around 6$-$7 Gyr, in
agreement with previous works using open clusters and field stars with precise
ages. Our research confirms that Ca II H \& K lines remain a useful
chromospheric evolution tracer until stars reach ages of at least 6$-$7 Gyr. We
found an evidence that, for the most homogeneous set of old stars, the
chromospheric activity indices seem to continue decreasing after the solar age
towards the end of the main-sequence. Our results indicate that a significant
part of the scatter observed in the age-activity relation of solar twins can be
attributed to stellar cycle modulations effects. The Sun seems to have a normal
activity level and variability for its age.
