Abstarct: This ex post facto causal-comparative study compared vocabulary size and reading comprehension between Grade 9 students who began formal English instruction in Grade 1 (early starters) and those who began in Grade 5 (late starters) in Iraqi public schools. Two research questions guided the study: (a) What is the difference in vocabulary size between early and late English starters? and (b) What is the difference in reading comprehension between early and late English starters? Two directional hypotheses predicted that early starters would have larger vocabulary sizes and higher reading comprehension than late starters. The sample consisted of 165 Grade 9 students selected through stratified cluster sampling from six public secondary schools in the Al-Karkh and Al-Rusafa districts of Baghdad Governorate, Iraq. Three schools represented the early-start model (English instruction began in Grade 1), and three schools represented the late-start model (English instruction began in Grade 5). Within each school, one intact Grade 9 class was randomly selected. Eighty-three students were early starters, and 82 students were late starters. All students completed a 100-item vocabulary size test (maximum score 100) measuring receptive vocabulary knowledge across three frequency levels, and a 28-item reading comprehension test (maximum score 28, reported as percentages) with three passages measuring literal, inferential, and interpretive comprehension. Data were analyzed using independent-samples t-tests with effect sizes (Cohen's d). Assumption checks (normality, homogeneity of variance) were performed and satisfied. The findings showed that early starters scored significantly higher on both measures. For vocabulary size, the early starter group (M = 76.37, SD = 7.18) outperformed the late starter group (M = 64.38, SD = 10.01), t(163) = 8.86, p < .001, one-tailed, d = 1.39. For reading comprehension, the early starter group (M = 81.23, SD = 8.94) outperformed the late starter group (M = 70.91, SD = 11.23), t(163) = 6.55, p < .001, one-tailed, d = 1.02. Both null hypotheses were rejected. The findings are consistent with previous international research and support the Critical Period Hypothesis and the Lexical Quality Hypothesis. The study provides evidence that earlier English instruction is associated with substantially larger vocabularies and higher reading comprehension by Grade 9 in the Iraqi public school context.