While it is well known that production of new neurons from neural stem/progenitor cells (NSC) in the dentate gyrus (DG) diminishes greatly by middle age, the phases and mechanisms of major age-related decline in DG neurogenesis are largely unknown. To address these issues, we first assessed DG neurogenesis in multiple age groups of Fischer 344 rats via quantification of doublecortin-immunopositive (DCX+) neurons and then measured the production, neuronal differentiation and initial survival of new cells in the subgranular zone (SGZ) of 4-, 12- and 24-month-old rats using four injections (one every sixth hour) of 5′-bromodeoxyuridine (BrdU), and BrdU–DCX dual immunostaining. Furthermore, we quantified the numbers of proliferating cells in the SGZ of these rats using Ki67 immunostaining. Numbers of DCX+ neurons were stable at 4–7.5 months of age but decreased progressively at 7.5–9 months (41% decline), 9–10.5 months (39% decline), and 10.5–12 months (34% decline) of age. Analyses of BrdU+ cells at 6 h after the last BrdU injection revealed a 71–78% decline in the production of new cells per day between 4-month-old rats and 12- or 24-month-old rats. Numbers of proliferating Ki67+ cells (putative NSCs) in the SGZ also exhibited similar (72–85%) decline during this period. However, the extent of both neuronal differentiation (75–81%) and initial 12-day survival (67–74%) of newly born cells was similar in all age groups. Additional analyses of dendritic growth of 12-day-old neurons revealed that newly born neurons in the aging DG exhibit diminished dendritic growth compared with their age-matched counterparts in the young DG. Thus, major decreases in DG neurogenesis occur at 7.5–12 months of age in Fischer 344 rats. Decreased production of new cells due to proliferation of far fewer NSCs in the SGZ mainly underlies this decline.