Kidney function, assessed as estimated glomerular filtration rate (eGFR), declines by age. In clinical practice, it is important to understand whether a person has an eGFR value as expected given the person’s age, or whether the value is lower than expected and potentially a reason for concern. Although chronic kidney disease is defined as eGFR <60 ml/min per 1.73 m2, the question arises whether a value of, for example, 58 ml/min per 1.73 m2 for an 80-year-old person is indicative of disease or age appropriate. We collected data from >12,000 individuals, aged 25 to 95 years, from population-based German studies. We provide age-specific reference values for eGFR usable in clinical practice to answer this question. Longitudinal information on eGFR decline was analyzed to also provide reference values for eGFR-decline by risk profile groups. Advanced regression models were applied for these analyses. Our results are interpretable and usable to help in clinical routine.