  The nuclear stellar disc (NSD) is a flat dense stellar structure at the heart
of the Milky Way. Recent work has shown that analogous structures are common in
the nuclei of external spiral galaxies, where there is evidence of an age
gradient that indicates that they form inside-out. However, the
characterisation of the age of the NSD stellar population along the line of
sight is still missing due to its extreme source crowding and the high
interstellar extinction towards the Galactic centre. We aim to characterise the
age of the stellar population at different average Galactocentric NSD radii to
investigate for the first time the presence of an age gradient along the line
of sight. We selected two groups of stars at different NSD radii via their
different extinction and proper motion distribution. We analysed their stellar
population by fitting their de-reddened $K_s$ luminosity functions with a
linear combination of theoretical models. We find significant differences in
the stellar population at different NSD radii, indicating the presence of an
age gradient along the line of sight. Our sample from the closest edge of the
NSD contains a significant fraction ($\sim40 \%$ of its total stellar mass) of
intermediate-age stars (2-7 Gyr) that is not present in the sample from stars
deeper inside the NSD, in which $\sim90 \%$ of the stellar mass is older than 7
Gyr. Our results suggest that the NSD age distribution is similar to the one
found in external galaxies and they imply that bar-driven processes observed in
external galaxies are similarly at play in the Milky Way.
