  The cooling of neutron stars (hereafter NS) has the potential to reveal
important features of superdense matter. Their surface temperatures are known
for a fair sample of NS with ages $\leq 10^{6} \, {\it{yr}}$, and with a few
exceptions, can be accommodated by standard cooling mechanisms (neutrino+photon
emission without internal heating). However, for the older objects it is
necessary to consider some internal heating to explain surface temperatures
higher than expected. We revisit in this paper the kinetic heating by fermionic
dark matter, rotochemical heating and magnetic field decay. We found that NS
slightly older than $\sim 10^{6} \, {\it{yr}}$ can be explained by them, but
the older ``black widow'' systems are much hotter than the values predicted by
these three mechanisms, pointing towards a yet unknown heating factor for old
NS.
