  It is generally accepted that the PSR B1509-58 is associated with the
supernova remnant (SNR) MSH 15-52 (G 320.4-01.2). The spin-down age of the
pulsar is \simeq 1700 years, while the size and the general appearance of the
SNR suggest that this system is much older. A few possible explanations of this
discrepancy have been put forward. We offer an alternative one and suggest that
the high spin-down rate of the pulsar characterizes only a relatively short
period of its (present) spin history, and that the enhanced braking torque is
connected with the interaction between the pulsar's magnetosphere and the dense
matter of a circumstellar clump (created during the late evolutionary stages of
the supernova (SN) progenitor star). Our suggestion implies that the "true" age
of PSR B1509-58 could be much larger than the spin-down age, and therefore the
SNR MSH 15-52 is a middle-aged remnant similar to the Vela SNR (G 263.9-3.3).
We also suggest that the dense (neutral) gas of the circumstellar clump could
be responsible for the enhanced neutral hydrogen absorption towards PSR
B1509-58, and that the optical emission of an optical counterpart for PSR
B1509-58 should rather be attributed to a bow shock around this pulsar than to
the pulsar itself.
