This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Renal physiology, Subject name: Physiology
Glomerular filtration rate stas declining at what age? 
Select and explain. 
A. 10-15 years, B. 30-40 years, C. 45-55 years, D. 55-65 years
B: 30-40 years
Answer: B: 30-40 years 
Explanation: Glomerular filtration rate (GFR) is the volume of fluid filtered from the renal (kidney) glomerular capillaries into the Bowman&;s capsule per unit time.The normal range of GFR, adjusted for body surface area, is 100-130 ml/min/1.73m2 in men and women. In children, GFRmeasured by inulin clearance is 110 ml/min/1.73m2 until 2 years of age in both sexes, and then it progressively decreases. After age 40, GFR decreases progressively with age, by about 0.4 - 1.2 mL/min per yearRef: Ganong&;s review of medical physiology; 24th edition; page no; 679