We present the results from multifrequency VLBA observations from 327MHz to 8.4GHz of the gigahertz-peaked spectrum radio source PKS1518+047 (4C04.51) aimed at studying the spectral index distribution across the source. Further multifrequency archival Very Large Array 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 subcomponents. 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 γ <= 600 are still contributing to the radio spectrum, while electrons with higher energies have almost been completely depleted. The source radiative lifetime we derived is 2700 +/- 600years. Considering the best fit to the overall spectrum, we find that the time in which the nucleus has not been active represents almost 20 per cent of the whole source lifetime, indicating that the source was 2150 +/- 500 year old when the radio emission switched off.