  Aims: The knowledge of the properties of the youngest radio sources is very
important in order to trace the earliest phase of the evolution of the radio
emission. RXJ1459+3337, with its high turnover frequency (~25 GHz) provides a
unique opportunity to study this class of extreme objects. Methods:
High-sensitivity multi-frequency VLA observations have been carried out to
measure the flux-density with high accuracy, while multi-frequency VLBA
observations were performed, aimed at determining the pc-scale structure.
Archival ROSAT data have been used to infer the X-ray luminosity. Results: The
comparison between our new VLA data and those available in the literature shows
a steady increment of the flux-density in the optically-thick part of the
spectrum and a decrement of the turnover frequency. In the optically-thin
regime, the source flux density has already started to decrease. Such a
variability can be explained in terms of an adiabatically-expanding homogeneous
radio component. The frequency range spanned by our VLBA observations, together
with the resolution achieved, allows us to determine the source size and the
turnover frequency, and then to derive the magnetic field directly from these
observable quantities. The value obtained in this way is in good agreement with
that computed assuming equipartition condition. A similar value is also
obtained by comparing the radio and X-ray luminosities.
