Designers of helmet- and head-mounted displays (HMDs) often assume that monocular devices improve operator performance relative to binocular devices by increasing field of view and by allowing two tasks to be performed simultaneously (one by each eye). To test these assumptions, we implemented a modified useful field of view (UFOV) paradigm in which subjects localized a peripheral target along give meridians within a semicircular region while simultaneously performing a central task. The tasks were either presented to the same eye or to different eyes (simulating a monocular HMD). Because previous research has established age-related changes in the UFOV, the present study investigated the performance of middle-aged observers and compared it to results obtained from young observers in an earlier study. In general, middle-aged observers made more peripheral target localization errors than young observers, indicating an overall constriction of the UFOV. The dependence of localization performance on viewing condition, peripheral distractors and central task, however, was the same for both age groups. Most notably, there was no difference in performance as a function of viewing condition. Thus, these findings do not support the assumption that dividing attention between two eyes allows dual tasks to be performed more efficiently than when attention is divided within the visual field of one eye.