Numerosity judgment involves determining the number of items, which highly correlates with mathematical achievement. This study investigated age-related differences on numerosity judgment among middle-level elementary students and college students in terms of strategy use and problem-solving efficiency determined by participants’ eye movements. Stimuli were grids consisting of 7 x 7 units which were either “on” (yellow block) or “off” (blank). Participants were asked to determine the number of yellow blocks. Results showed that given energy- and time- consuming, third graders consistently adopted “addition strategy” on larger numerosity trials. For strategy adaptiveness, adults outperformed younger peers, most of whom starting using “subtraction strategy” on critical and larger trials (e.g., 25 and larger numerosities). Regarding efficiency, adults determined numerosities more efficiently than elementary students, because adults’ eye fixations were significantly fewer than those of younger groups, while only marginal difference on the number of fixations was observed for the two younger groups.