  HARPS spectra with S/N > 600 for 21 solar twin stars are used to determine
very precise (sigma ~ 0.01 dex) differential abundances of C, O, Na, Mg, Al,
Si, S, Ca, Ti, Cr, Fe, Ni, Zn, and Y in order to see how well [X/Fe] is
correlated with elemental condensation temperature, Tc. In addition, precise
(sigma < 0.8 Gyr) stellar ages are obtained by interpolating between
Yonsei-Yale isochrones in the logg - Teff diagram. It is confirmed that the
ratio between refractory and volatile elements is lower in the Sun than in most
of the solar twins, but for many stars, the relation between [X/Fe] and Tc is
not well defined. For several elements there is, instead, an astonishingly
tight correlation between [X/Fe] and stellar age with amplitudes up to 0.2 dex
over an age interval of 8 Gyr in contrast to the lack of correlation between
[Fe/H] and age. While [Mg/Fe] increases with age, the s-process element yttrium
shows the opposite behavior so that [Y/Mg] can be used as a sensitive
chronometer for Galactic evolution. [Na/Fe] and [Ni/Fe] are not well correlated
with stellar age, but define a tight Ni-Na relation similar to that previously
found for more metal-poor stars. These results provide new constraints on
supernovae yields and Galactic evolution. Furthermore, it is found that the C/O
ratio evolves very little with time, which is of interest for discussions of
the composition of exoplanets.
