  The age-velocity dispersion relation is an important tool to understand the
evolution of the disc of the Andromeda galaxy (M31) in comparison with the
Milky Way. We use Planetary Nebulae (PNe) to obtain the age-velocity dispersion
relation in different radial bins of the M31 disc. We separate the observed PNe
sample based on their extinction values into two distinct age populations. The
observed velocities of our high- and low-extinction PNe, which correspond to
higher and lower mass progenitors respectively, are fitted in de-projected
elliptical bins to obtain their rotational velocities, $V_{\phi}$, and
corresponding dispersions, $\rm\sigma_{\phi}$. We assign ages to the two PNe
populations by comparing central-star properties of an archival sub-sample of
PNe, having models fitted to their observed spectral features, to stellar
evolution tracks. For the high- and low-extinction PNe, we find ages of
$\sim2.5$ Gyr and $\sim4.5$ Gyr respectively, with distinct kinematics beyond a
deprojected radius R$\rm_{GC}= 14$ kpc. At R$\rm_{GC}$=17--20 kpc, which is the
equivalent distance in disc scale lengths of the Sun in the Milky Way disc, we
obtain $\rm\sigma_{\phi,~2.5~Gyr}= 61\pm 14$ km s$^{-1}$ and
$\rm\sigma_{\phi,~4.5~Gyr}= 101\pm 13$ km s$^{-1}$. The age-velocity dispersion
relation for the M31 disc is obtained in two radial bins, R$\rm_{GC}$=14--17
and 17--20 kpc. The high- and low-extinction PNe are associated with the young
thin and old thicker disc of M31 respectively, whose velocity dispersion values
increase with age. These values are almost twice and thrice that of the Milky
Way disc stellar population of corresponding ages. From comparison with
simulations of merging galaxies, we find that the age-velocity dispersion
relation in the M31 disc measured using PNe is indicative of a single major
merger that occurred 2.5 -- 4.5 Gyr ago with an estimated merger mass ratio
$\approx$ 1:5.
