Final answer:
Kidney function declines by approximately 50% from the age of 20 to 90 years, with a decrease in glomerular filtration rate as the primary marker of this decline.
Step-by-step explanation:
Kidney function decline is a critical aspect of aging. Studies suggest that kidney function declines by approximately 50% between the ages of 20 and 90 years. This decline is due to a gradual loss in glomerular filtration rate (GFR), which is a measure of how well the kidneys are filtering waste from the blood. A healthy, young adult typically has a GFR of around 90 mL/min/1.73 m² or higher.
With age, the GFR declines, indicating reduced kidney function. Chronic kidney disease (CKD) stages are defined based on GFR values, with Stage 3 representing a moderate reduction in GFR (30-59 mL/min/1.73 m²) and Stage 5 indicating established kidney failure (GFR <15 mL/min/1.73 m²). Conditions such as diabetes mellitus and high blood pressure can exacerbate the natural decline in kidney function.