  A new method is presented to compute age estimates from theoretical
isochrones using temperature, luminosity and metallicity data for individual
stars. Based on Bayesian probability theory, this method avoids the systematic
biases affecting simpler strategies, and provides reliable estimates of the age
probability distribution function for late-type dwarfs. This method is
especially relevant for G dwarfs in the 3-15 Gyr range of ages, crucial to the
study of the chemical and dynamical history of the Galaxy.
  We apply our method to the classic sample of Edvardsson et al. (1993), who
derived the age-metallicity relation (AMR) of a sample of 189 field dwarfs with
precisely determined abundances. Using new parallax, temperature and
metallicity data, our age determination for the Edvardsson et al. sample
indicates that the intrinsic dispersion in the AMR is at most 0.15 dex and
probably lower. In particular, we show that old, metal-rich objects ([Fe/H]\sim
0.0 dex, age > 5 Gyr) and young, metal-poor objects ([Fe/H]<-0.5 dex, age < 6
Gyr) in many observed AMR plots are artifacts caused by a too simple treatment
of the age determination, and that the Galactic AMR is monotonically increasing
and rather well-defined. Incidentally, our results tend to restore confidence
in the method of age determination from chromospheric activity for field
dwarfs.
