THE rotation rate of a pulsar is thought to decrease with time according to a simple power law, with a ’braking index’ equal to 3 if rotational energy is lost through radiation from a dipolar magnetic field(1-3). The age of the pulsar can accordingly be determined simply by measuring the current rotation rate, and its current rate of change. Here we report an analysis of the rotation rate of the Vela pulsar as observed over 25 years. We find that the braking index is 1.4+/-0.2, suggesting that the braking cannot be attributed entirely to radiation from a constant magnetic dipole but is probably due to a changing, magnetic moment or effective moment of inertia. Taken at face value, the result implies that the Vela pulsar may be much older than previously thought, and that inferred velocities of the supernova ejecta(4) and an X-ray jet from the pulsar(5) are correspondingly reduced. If other young pulsars associated with supernova remnants have similarly low braking indices, then they too may be much older than believed, thereby reducing their estimated velocities.