  The "characteristic age" of a pulsar usually is considered to approximate its
true age, but this assumption has led to some puzzling results, including the
fact that many pulsars with small characteristic ages have no associated
supernova remnants. The pulsar B1757-24 is located just beyond the edge of a
supernova remnant; the properties of the system indicate that the pulsar was
born at the centre of the remnant, but that it has subsequently overtaken the
expanding blast-wave. With a characteristic age of 16,000 yr, this implies an
expected proper motion by the pulsar of 63-80 milliarcsec per year. Here we
report observations of the nebula surrounding the pulsar which limit its proper
motion to less than 25 mas/yr, implying a minimum age of 39,000 yr. A more
detailed analysis argues for a true age as great as 170,000 yr, significantly
larger than the characteristic age. From this result and other discrepancies
associated with pulsars, we conclude that characteristic ages seriously
underestimate the true ages of pulsars.
