Constraining the spectral age of very asymmetric CSOs - Evidence of the influence of the ambient medium

Aims. We constrain the spectral ages for two very asymmetric Compact Symmetric Objects (CSO) from the B3-VLA-CSS sample, and we investigate the role of the ambient medium potentially able to influence the individual source evolution. Methods. Multi-frequency VLBA observations have been carried out to study the distribution of the break frequency of the spectra across different regions of each source. Results. From the analysis of synchrotron spectra and assuming an equipartition magnetic field, we find radiative ages of about 5 x 10(3) and 5 x 10(4) years for B0147+400 and B0840+424, respectively. The derived individual hot-spot advance speed is in the range between 0.03c and 0.3c, in agreement with kinematic studies carried out on other CSOs. The very asymmetric morphology found in both sources is likely related to an inhomogeneous ambient medium in which the sources are growing, rather than to different intrinsic hot-spot pressures on the two sides.