Technical Abstract: Near-infrared spectroscopy (NIRS) was recently applied to age-grade and differentiate laboratory reared Anopheles gambiae sensu strico and Anopheles arabiensis sibling species of Anopheles gambiae sensu lato. In this study, we report further on the accuracy of this tool in simultaneously estimating the age class and differentiating the morphologically indistinguishable An. gambiae s.s. and An. arabiensis from semi-field releases and wild populations. Nine different ages (1, 3, 5, 7, 9, 11, 12, 14, 16 d) of An. arabiensis and eight different ages (1, 3, 5, 7, 9, 10, 11, 12 d) of An. gambiae s.s. maintained in 250 x 60 x 40 cm cages within a semi-field large-cage system and 105 female wild An. gambiae s.l., were included in this study. NIR classified female An. arabiensis and An. gambiae s.s. maintained in semi field cages as < 7 d old or = 7 d old with 89% (n=377) and 78 % (n=327) accuracy, respectively and differentiated them with 89% (n=704) accuracy. Wild caught An. gambiae s.l. were identified with 90% accuracy (n=105) whereas their predicted age were consistent with the expected mean chronological ages of the physiological age categories determined by dissections. These findings have importance for monitoring control programmes where reduction in the proportion of older mosquitoes that have the ability to transmit malaria is an important outcome.