  Measured from high-resolution spectra, we present Ca II K line chromospheric
fluxes in 35 G dwarf stars of 5 open clusters to determine the
age-activity-rotation relationship from the young Hyades and Praesepe (0.6 Gyr)
to the old M 67 (~ 4.5 Gyr) through the two intermediate age clusters IC 4651
and NGC 3680 (~ 1.7 Gyr). Our data show that a dramatic decrease in
chromospheric activity takes place among solar stars between the Hyades and the
IC4651 age, in about 1 Gyr. Afterwards, activity remains virtually constant for
more than 3 Gyr. We briefly investigate the impact of this result on the age
determinations of field G dwarfs in the solar neighborhood; the two main
conclusions are that a consistent group of 'young' stars (i.e. as active as
Hyades stars) is present, and that it is virtually impossible to give accurate
chromospheric ages for stars older than ~ 2 Gyr. The observed abrupt decline in
activity explains very well the Vaughan-Preston gap.
