There is evidence that some brain regions show age-associated volume decline and that men undergo more accelerated cerebral aging than women. However, limited information is available on age-associated changes during young adulthood. The authors performed quantitative magnetic resonance imaging in 116 healthy young adults (57 men, 59 women; age range: 18-49 years) to evaluate the relationship of age and sex with volumes of frontal and temporal regions, including selected limbic structures and the basal ganglia. Regardless of sex, increasing age was moderately associated with decrease in total gray matter (GM) and mildly with increase in sulcal cerebrospinal fluid (CSF). Associations of age with reduced GM volume were observed in all frontal and temporal cortical regions and some basal ganglia structures, but were generally less prominent in subcortical regions. The associations were stronger for men than women in the dorsolateral prefrontal cortex. Thus, in young, healthy adults, age-associated changes are selective as well as sex-specific, with men experiencing greater volume decrement across age-groups than women, particularly in the dorsolateral prefrontal regions. Reduced GM and increased CSF in this age range suggests that the aging process is a continuum, with changes evident before senescence. Thus, the biological changes commonly attributed to "aging" do, in fact, begin much earlier in the life-cycle.