  We investigate the radio spectral energy distributions (SEDs) of young
star-forming galaxies and how they evolve with time. The duration and
luminosity of the nonthermal radio emission from supernova remnants (SNRs) are
constrained by using the observational radio SEDs of SBS 0335-052 and I Zw 18,
which are the two lowest-metallicity blue compact dwarf galaxies in the nearby
universe. The typical radio ``fluence'' for SNRs in SBS 0335-052, that is the
radio energy emitted per SNR over its radiative lifetime, is estimated to be
$\sim 6$--$22\times 10^{22} {\rm W Hz^{-1} yr}$ at 5 GHz. On the other hand,
the radio fluence in I Zw 18 is $\sim 1$--$3\times 10^{22} {\rm W Hz^{-1} yr}$
at 5 GHz. We discuss the origin of this variation and propose scaling relations
between synchrotron luminosity and gas density. We have also predicted the time
dependence of the radio spectral index and of the spectrum itself, for both the
``active'' (SBS 0335-052) and ``passive'' (I Zw 18) cases. These models enable
us to roughly age date and classify radio spectra of star-forming galaxies into
active/passive classes. Implications for high-z galaxy evolution are also
discussed.
