  Abundances of light elements in dwarf stars of different ages are important
constraints for stellar yields, Galactic chemical evolution and exoplanet
chemical composition studies. We have measured C and N abundances and
$^{12}$C/$^{13}$C ratios for a sample of 63 solar twins spanning a wide range
in age, based on spectral synthesis of a comprehensive list of CH\,A-X and
CN\,B-X features using HARPS spectra. The analysis of 55 thin disc solar twins
confirms the dependences of [C/Fe] and [N/Fe] on [Fe/H]. [N/Fe] is investigated
as a function of [Fe/H] and age for the first time for these stars. Our derived
correlation [C/Fe]-age agrees with works for solar-type stars and solar twins,
but the [N/Fe]-age correlation does not. The relations [C,N/Fe]-[Fe/H] and
[C,N/Fe]-age for the solar twins lay under-solar. $^{12}$C/$^{13}$C is found
correlated with [Fe/H] and seems to have decreased along the evolution of the
local thin disc. Predictions from chemical evolution models for the solar
vicinity corroborate the relations [C,N/Fe]-[Fe/H], $^{12}$C/$^{13}$C-age and
[N/O]-[O/H], but do not for the $^{12}$C/$^{13}$C-[Fe/H] and [C/O]-[O/H]
relations. The N/O ratio in the Sun is placed at the high end of the
homogeneous distribution of solar twins, which suggests uniformity in the N-O
budget for the formation of icy planetesimals, watery super-earths and giant
planets. C and N had different nucleosynthetic origins along the thin disc
evolution, as shown by the relations of [C/N], [C/O] and [N/O] against [O/H]
and age. [C/N] and [C/O] are particularly observed increasing in time for solar
twins younger than the Sun.
