  With the successful launch and commissioning of JWST we are now able to
routinely spectroscopically probe the rest-frame optical emission of galaxies
at $z>6$ for the first time. Amongst the most useful spectral diagnostics used
in the optical is the Balmer/4000~\AA\ break; this is, in principle, a
diagnostic of the mean ages of composite stellar populations. However, the
Balmer break is also sensitive to the shape of the star formation history, the
stellar (and gas) metallicity, the presence of nebular continuum emission, and
dust attenuation. In this work we explore the origin of the Balmer/4000~\AA\
break using the SYNTHESIZER synthetic observations package. We then make
predictions of the Balmer/4000~\AA\ break using the First Light and
Reionisation Epoch Simulations (FLARES) at $5<z<10$. We find that the average
break strength weakly correlates with stellar mass and rest-frame far-UV
luminosity, but that this is predominantly driven by dust attenuation. We also
find that break strength provides a weak diagnostic of the age but performs
better as a means to constrain star formation and stellar mass, alongside the
UV and optical luminosity, respectively.
