  Early JWST spectroscopic campaigns have confirmed the presence of strong [O
III] line-emitting galaxies in the redshift interval $7<z<9$. Although deduced
earlier from Spitzer photometry as indicative of young stellar populations,
some studies suggested the relevant photometric excesses attributed to [O III]
emission could, in part, be due to Balmer breaks arising from older stars. We
demonstrate that this is likely the case by exploiting medium-band
near-infrared JWST photometry in the Hubble Ultra Deep Field. We locate a
sample of 6 galaxies with redshifts 8.2$<z<$8.6 for which the relevant
medium-band filters enables us to separate the contributions of [O III]
emission and a Balmer break, thereby breaking earlier degeneracies of
interpretation. The technique is particularly valuable since it provides
photometric redshifts whose precision, $\Delta\,z\simeq\,\pm0.08$, approaches
that of spectroscopic campaigns now underway with JWST. Although some sources
are young, a third of our sample have prominent Balmer breaks consistent with
stellar ages of $\geq$150 Myr. Our results indicate that even intense [O III]
emitters experienced episodes of earlier star formation to $z\sim$10 and
beyond, as is now being independently deduced from direct detection of the
progenitors of similar systems.
