  Connecting the gas in HII regions to the underlying source of the ionizing
radiation can help us constrain the physical processes of stellar feedback and
how HII regions evolve over time. With PHANGS$\unicode{x2013}$MUSE we detect
nearly 24,000 HII regions across 19 galaxies and measure the physical
properties of the ionized gas (e.g. metallicity, ionization parameter,
density). We use catalogues of multi-scale stellar associations from
PHANGS$\unicode{x2013}$HST to obtain constraints on the age of the ionizing
sources. We construct a matched catalogue of 4,177 HII regions that are clearly
linked to a single ionizing association. A weak anti-correlation is observed
between the association ages and the H$\alpha$ equivalent width EW(H$\alpha$),
the H$\alpha$/FUV flux ratio and the ionization parameter, log q. As all three
are expected to decrease as the stellar population ages, this could indicate
that we observe an evolutionary sequence. This interpretation is further
supported by correlations between all three properties. Interpreting these as
evolutionary tracers, we find younger nebulae to be more attenuated by dust and
closer to giant molecular clouds, in line with recent models of
feedback-regulated star formation. We also observe strong correlations with the
local metallicity variations and all three proposed age tracers, suggestive of
star formation preferentially occurring in locations of locally enhanced
metallicity. Overall, EW(H$\alpha$) and log q show the most consistent trends
and appear to be most reliable tracers for the age of an HII region.
