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Global very long baseline interferometry (VLBI) observations at low frequencies are of great importance since they allow us to test the oldest part of the relativistic electron populations in young radio sources. At 327 MHz ((Formula presented.) 92 cm), the most compact regions are self-absorbed, and the number of cores detected is very small. This could provide useful measurements of the magnetic field in the relativistic plasma, to be compared with the equipartition field derived from minimum energy arguments. Partial self-absorption takes place also in hot-spots. However, disentangling the hot-spot emission from that of the lobe turned out to be rather difficult. On the other hand, a clearer and more complete characterization of the volumes occupied by the magnetized relativistic plasma and its total energy content can be obtained at 327 MHz. In turn, an independent estimate of the source age can be inferred, on the basis of simple assumptions on the jet power. Here, we show that the ages derived in this way (between (Formula presented.) and (Formula presented.) year) are consistent with the radiative ages determined from the break frequency in the radio spectrum.