  The high-frequency integrated spectra of Compact Steep Spectrum (CSS) sources
show breaks with a moderate spectral steepening well fitted by continuous
injection synchrotron spectra. In lobe-dominated CSS sources the radiative ages
deduced by the synchrotron theory are in the range of up to 0.1 Myears, if
equipartition magnetic fields are assumed. These radiative ages are well
correlated with the source size indicating that the CSS sources are young. In
order to maintain the frustration scenario, in which the sources' lifetimes are
about 10 Myears, their equipartition magnetic field would be systematically
decreased by a factor of more than 20. To complete the sample used in this
work, we conducted observations at 230 GHz with the IRAM 30-m telescope of
those sources which did not have such high-frequency observations up to now.
