Hyperphosphatemia is pivotal in some complications secondary to kidney dysfunction. Current guidelines suggest that hyperphosphatemia due to kidney dysfunction develops only when kidney function is reduced to <50%. This paper deals with the relationship of age to phosphatemia and with the possible influences of this relationship on hyperphosphatemia due to kidney dysfunction. In pediatric age phosphatemia decreases during growth. A recent epidemiological study showed a decrease in phosphatemia with age among adults also. This decrease differs between men and women, being continuous in men but not women because of a transitory increase in phosphatemia during menopause. Data also show that age-associated differences in phosphatemia among adults are explained by differences in the maximum reabsorption of phosphate in the renal proximal tubule (TmP/GFR). Other studies suggest that the opposite influences on TmP/GFR of growth hormone (stimulation) and estrogens (inhibition) are the determinants of the changes in TmP/GFR and phosphatemia associated with age. The inverse relationship of age with phosphatemia leads to the hypothesis that, in the presence of a disorder increasing phosphatemia, the prevalence of hyperphosphatemia would be higher in young adults than in elderly people, who have lower phosphatemia in health. A large clinical study supports this hypothesis, showing that hyperphosphatemia secondary to kidney dysfunction is approximately four times higher at age <65 than at age >65 years. Data suggest that the relation between kidney function and phosphatemia should be re-evaluated considering possible confounding due to age.