  Aims. The main goal of this work is to date young open clusters using
$\delta$ Sct stars. Seismic indices such as the large separation and the
frequency at maximum power can help to constrain the models to better
characterise the stars. We propose a reliable method to identify some radial
modes, which gives us greater confidence in the constrained models. Methods. We
extract the frequency content of a sample of $\delta$ Sct stars belonging to
the same open cluster. We estimate the low-order large separation by means of
different techniques and the frequency at maximum power for each member of the
sample. We use a grid of models built with the typical parameters of $\delta$
Sct stars, including mass, metallicity and rotation as independent variables,
and determine the oscillation modes. We select the observed frequencies whose
ratios match those of the models. Once we find a range of radial modes matching
the observed frequencies, mainly the fundamental mode, we add it to the other
seismic parameters to derive the stellar age. Assuming star groups have similar
chemistry and age, we estimate their mean age by computing a weighted
probability density function fit to the age distribution of the seismically
constrained models. Results. We estimate the age of Trumpler 10 to be
$30_{-20}{+30}$ Myr, and that of Praesepe to be $580 \pm 230$ Myr. In this
latter case, we find two apparent populations of $\delta$ Sct stars in the same
cluster, one at $510 \pm 140$ Myr and another at $890 \pm 140$ Myr. This may be
due to two different formation events, different rotational velocities of the
members in our sample of stars (as rapid rotation may modify the observed large
separation), or to membership of unresolved binary systems.
