  With JWST, new opportunities to study the evolution of galaxies in the early
Universe are emerging. Spitzer constraints on rest-optical properties of
$z\gtrsim7$ galaxies demonstrated the power of using galaxy stellar masses and
star formation histories (SFHs) to indirectly infer the cosmic star formation
history. However, only the brightest individual $z\gtrsim8$ objects could be
detected with Spitzer, making it difficult to robustly constrain activity at
$z\gtrsim10$. Here, we leverage the greatly improved rest-optical sensitivity
of JWST at $z\gtrsim8$ to constrain the ages of seven UV-bright
($M_{UV}\lesssim-19.5$) galaxies selected to lie at $z\sim8.5-11$, then
investigate implications for $z\gtrsim15$ star formation. We infer the
properties of individual objects with two spectral energy distribution
modelling codes, then infer a distribution of ages for bright $z\sim8.5-11$
galaxies. We find a median age of $\sim20$ Myr, younger than that inferred at
$z\sim7$ with a similar analysis, consistent with an evolution towards larger
specific star formation rates at early times. The age distribution suggests
that only $\sim3$ percent of bright $z\sim8.5-11$ galaxies would be similarly
luminous at $z\gtrsim15$, implying that the number density of bright galaxies
declines by at least an order of magnitude between $z\sim8.5-11$ and $z\sim15$.
This evolution is challenging to reconcile with some early JWST results
suggesting the abundance of bright galaxies does not significantly decrease
towards very early times, but we suggest this tension may be eased if young
stellar populations form on top of older stellar components, or if bright
$z\sim15$ galaxies are observed during a burst of star formation.
