We report a longitudinal comprehension study of (long) passive constructions in two native-Spanish child groups differing by age of initial exposure to L2 English (young group: 3;0–4;0; older group: 6;0–7;0), where amount of input, L2 exposure environment, and socioeconomic status are controlled. Data from a forced-choice task show that both groups comprehend active sentences, not passives, initially (after 3·6 years of exposure). One year later, both groups improve, but only the older group reaches ceiling on both actives and passives. Two years from initial testing, the younger group catches up. Input alone cannot explain why the younger group takes five years to accomplish what the older group does in four. We claim that some properties take longer to acquire at certain ages because language development is partially constrained by general cognitive and linguistic development (e.g. de Villiers, 2007; Long & Rothman, 2014; Paradis, 2008, 2010, 2011; Tsimpli, 2014).