I found these so far (BOLDING mine)

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2744545/

The process of glomerular filtration of plasma fluid has been known for over 160 years and the measurement of the rate of its formation (glomerular filtration rate, GFR) has been possible for over 80 years. Studies conducted in the 1930's to the 1950's clearly established that GFR declines, perhaps inexorably, with normal ageing, usually beginning after 30–40 years of age. The rate of decline may accelerate after age 50–60 years. This decline appears to be a part of the normal physiologic process of cellular and organ senescence and is associated with structural changes in the kidneys