  Extended main-sequence turn-offs (eMSTO) are a commonly observed property of
young clusters. A global theoretical interpretation for the eMSTOs is still
lacking, but stellar rotation is considered a necessary ingredient to explain
the eMSTO. We aim to assess the importance of core-boundary and envelope mixing
in stellar interiors for the interpretation of eMSTOs in terms of one coeval
population. We construct isochrone-clouds based on interior mixing profiles of
stars with a convective core calibrated from asteroseismology of isolated
galactic field stars. We fit these isochrone-clouds to the measured eMSTO to
estimate the age and core mass of the stars in the two young clusters NGC 1850
and NGC 884, assuming one coeval population and fixing the metallicity to the
one measured from spectroscopy. We assess the correlations between the interior
mixing properties of the cluster members and their rotational and pulsation
properties. We find that stellar models based on asteroseismically-calibrated
interior mixing profiles lead to enhanced core masses of eMSTO stars and can
explain a good fraction of the observed eMSTOs of the two considered clusters
in terms of one coeval population of stars, with similar ages to those in the
literature, given the large uncertainties. The rotational and pulsation
properties of the stars in NGC 884 are not sufficiently well known to perform
asteroseismic modelling, as it is achieved for field stars from space
photometry. The stars in NGC 884 for which we have vsini and a few pulsation
frequencies show no correlation between these properties and the core masses of
the stars that set the cluster age. Future cluster space asteroseismology may
allow to interpret the values of the core masses in terms of the physical
processes that cause them, based on the modelling of the interior mixing
profiles for the individual member stars with suitable identified modes.
