I hope it's OK to post an answer to my own question since it's community-wiki. Here are a couple of things I found down this rabbit-hole.
Dean Simonton at UC Davis has done some work claiming that there is a slow age-related decline in quality and quantity of creative output, but the relevant variable is career age, not biological age. He also makes it clear that although he believes there is a clear aggregate trend, the individual variability is much greater than the aggregate variability. Furthermore, he attributes the decline mostly to factors other than biological aging.
Simonton, D. K. (1997). Creative productivity: A predictive and explanatory model of career trajectories and landmarks. Psychological Review, 104, 66-89.
This paper is behind a subscription paywall (but there is a link below in the comments), so instead I'm posting this link to the PowerPoint (sorry) of his 2005 talk at the Max Planck International Research Network on Aging:
I couldn't find a good sound bite from Simonton's paper. Here is a quote from Arne Dietrich's 2004 paper The cognitive neuroscience of creativity:
Simonton (1997) has convincingly demonstrated that
“creative productivity is a function of career age, not
chronological age” (p. 70). Although career age and
chronological age are highly correlated, latecomers to a
discipline show the same career trajectories and landmarks,
as well as conformity to the 10-year rule (Simonton,
1997, 2003). For instance, mathematicians peak on
average at 26.5 years of career age, while historians peak
at 38.5 (Simonton, 1997). Because prefrontal-dependent
mental functions do not significantly decline until old
age, the distinction between chronological and career age
can be accommodated as long as the creator’s career onset
is not at an advanced chronological age.