The aim of this review was to examine the evidence for age-related changes of the hypothalamic-pituitary-adrenal (HPA) axis in both physiological and pathological aging, on the basis of the many data in the literature, as well as of our personal findings. A statistically significant circadian rhythmicity of serum cortisol was maintained in elderly subjects, even if with a reduced amplitude of the 24 h fluctuations and a trend to an increase of the serum levels in the evening and at night-time, in comparison with young controls. Furthermore, an age-related impairment of HPA sensitivity to steroid feedback was present in elderly people. The occurrence of senile dementia amplified the changes already present in physiological aging. While the cortisol secretion was generally well maintained in aging, the adrenal production of dehydroepiandrosterone and of its sulfate (DHEAS) exhibited an age-related decline. Therefore, the cortisol/DHEAS molar ratio was significantly higher in elderly subjects and even more in demented ones, than in young controls. Due to the opposite effects of cortisol and DHEAS on the brain and particularly on the hippocampal region, the imbalance between glucocorticoids and androgens occurring in physiological and even more in pathological aging, may have adverse effects on the function of this region, whose key role in learning and memory is well known.