  We report on the X-ray properties of four rotation-powered pulsars with
characteristic ages in the range 0.3-5 Myr, derived from the analysis of
XMM-Newton archival observations. We found convincing evidence of thermal
emission only in the phase-averaged spectrum of PSR B0114+58, that is well
fitted by a blackbody with temperature kT = $0.17 \pm 0.02$ keV and emitting
radius R = $405_{-90}^{+110}$ m, consistent with the size of its polar cap. The
other three considered pulsars, PSR B0628-28, PSR B0919+06 and PSR B1133+16,
have phase-averaged spectra well described by single power-laws with photon
index {\Gamma} ~ 3. The 3{\sigma} upper limits on the bolometric luminosity of
a possible thermal component with temperature in the range ~ 0.05-2 keV are
L_bol $\leq 3.2 \times 10^{28}$ erg/s and L_bol $\leq 2.4 \times 10^{29}$
erg/s, for PSR B0628-28 and PSR B0919+06, respectively. On the other hand, we
found possible evidence that the pulsed emission of PSR B0628-28 is thermal.
Two absorption lines at ~0.22 keV and ~0.44 keV are detected in the spectrum of
PSR B1133+16. They are best interpreted as proton cyclotron features, implying
the presence of multipolar components with a field of a few $10^{13}$ G at the
neutron star polar caps. We discuss our results in the context of high-energy
emission models of old rotation-powered pulsars.
