The phosphatidylinositol 3-kinase (PI3K)/Akt pathway is important for tissue proliferation. Previously, we found that tissue regeneration after partial pancreatic resection was markedly attenuated in aged mice as compared to young mice and that this attenuation was because of an age-dependent reduction of PI3K/Akt signaling in the pancreatic acini; however, the mechanisms for the age-associated decline of pancreatic PI3K/Akt signaling remained unknown. To better delineate the mechanisms for the decreased PI3K/Akt activation with aging, age-associated changes in cell proliferation and PI3K/Akt signaling were investigated in the present study using in vitro primary pancreatic acinar cell cultures derived from young and aged mice. In response to treatment with insulin-like growth factor 1 (IGF-1), acinar cells from young but not aged mice showed increased activation of PI3K/Akt signaling and cell proliferation, indicating that intrinsic cellular mechanisms cause the age-associated changes in pancreatic acinar cells. We also found that the expression of PI3K p85α subunit, but not IGF-1 receptor or other PI3K subunits, was significantly reduced in pancreatic acinar cells from aged mice; this age-associated reduction of p85α was confirmed in both mouse and human pancreatic tissues. Finally, small interfering RNA (siRNA)-mediated knockdown of p85α expression in acinar cells from young mice resulted in markedly attenuated activation of PI3K/Akt downstream signaling in response to IGF-1. From these results, we conclude that exocrine pancreatic expression of PI3K p85α subunit is attenuated by aging, which is likely responsible for the age-associated decrease in activation of pancreatic PI3K signaling and acinar cell proliferation in response to growth-promoting stimuli.