We aimed to determine the role of age-related changes in the mammalian target of rapamycin (mTOR) activity in endothelial cell growth during retinal vascular development in mice. Mice were administered the mTOR inhibitor rapamycin as follows: (i) for 6 days from postnatal day 0 (P0) to P5, (ii) for 2 days on P6 and P7, and (iii) for 2 days on P12 and P13. For comparison, we examined the effects of KRN633, an inhibitor of vascular endothelial growth factor (VEGF) receptor tyrosine kinase, on retinal vascular development. The retinal vasculature and phosphorylated ribosomal protein S6 (pS6), a downstream indicator of mTOR activity, were evaluated using immunohistochemistry. Vascularization was delayed and capillary density was reduced in mice administered rapamycin from P0 to P5 compared to the vehicle-treated mice. Rapamycin administration on P6 and P7 decreased the vascular density but did not significantly delay the radial vascular growth. Rapamycin administration on P12 and P13 did not significantly affect the retinal superficial blood vessels. Immunoreactivity for pS6 was detected in both endothelial cells in the vascular front and non-vascular cells in the retinal parenchyma, and rapamycin markedly diminished the pS6 immunoreactivity. KRN633 administration on P0 and P1 completely inhibited retinal vascularization. The effects of KRN633 on retinal blood vessels decreased in magnitude in an age-dependent manner. These results suggest that the mTOR pathway in endothelial cells activated by VEGF contributes to physiologic vascular development, and that the mTOR pathway in endothelial cells is modulated in a postnatal age-dependent manner.