Serum creatinine and 15-minute phenolsulfonphthalein excretion levels were obtained on 1,000 male and female patients, ranging from 25 to 94 years of age. The renal function tests were used to estimate each patient's glomerular and tubular activity in a quantitative fashion. Correlation of kidney function with the patient's age revealed that one third of the patients 65 years or older had normal glomerular and tubular function. More than two thirds of the elderly individuals demonstrated normal glomerular function.
Lapides J, Zierdt D. Compatibility of Normal Renal Function With Aging. JAMA. 1967;201(10):778–779. doi:10.1001/jama.1967.03130100076026