  We present the results from multi-frequency VLBA observations from 327 MHz to
8.4 GHz of the gigahertz-peaked spectrum radio source PKS 1518+047 (4C 04.51)
aimed at studying the spectral index distribution across the source. Further
multi-frequency archival VLA data were analysed to constrain the spectral shape
of the whole source. The pc-scale resolution provided by the VLBA data allows
us to resolve the source structure in several sub-components. The analysis of
their synchrotron spectra showed that the source components have steep spectral
indices, suggesting that no supply/re-acceleration of fresh particles is
currently taking place in any region of the source. By assuming the
equipartition magnetic field of 4 mG, we found that only electrons with
$\gamma$ < 600, are still contributing to the radio spectrum, while electrons
with higher energies have been almost completed depleted. The source radiative
lifetime we derived is 2700+/-600 years. Considering the best fit to the
overall spectrum, we find that the time in which the nucleus has not been
active represents almost 20% of the whole source lifetime, indicating that the
source was 2150+/-500 years old when the radio emission switched off.
