To elucidate the pathophysiologic changes in the kidney due to aging, we used physiological, morphometric, and imaging techniques to quantify GFR and its determinants in a group of 24 older (≥55 years) compared to 33 younger (≤45 years) living donors. Mathematical modeling was used to estimate the glomerular filtration coefficients for the whole kidney (K_f) and for single nephrons (SNK_f), as well as the number of filtering glomeruli (N_FG). Compared to younger donors, older donors had a modest (15%) but significant depression of pre-donation GFR. Mean whole-kidney K_f, renocortical volume, and derived N_FG were also significantly decreased in older donors. In contrast, glomerular structure and SNK_f were not different in older and younger donors. Derived N_FG in the bottom quartile of older donors was less than 27% of median-derived N_FG in the two kidneys of younger donors. Nevertheless, the remaining kidney of older donors exhibited adaptive hyperfiltration and renocortical hypertrophy post-donation, comparable to that of younger donors. Thus, our study found the decline of GFR in older donors is due to a reduction in K_f attributable to glomerulopenia. We recommend careful monitoring for and control of post-donation comorbidities that could exacerbate glomerular loss.