The fundamental difference hypothesis (FDH; Bley-Vroman, 1989, 1990) contends that the nature of language in natives is fundamentally different from the nature of language in adult nonnatives. This study tests the FDH in two ways: (a) via second language (L2) poverty-of-the-stimulus (POS) problems (e.g., Schwartz & Sprouse, 2000) and (b) via a comparison between adult and child L2 learners, whose first language (L1) is the same, in terms of their developmental route (e.g., Schwartz, 1992, 2003). The phenomena under investigation are Korean wh-constructions with negative polarity items (NPIs). Korean has subject (S)-object (O)-verb (V) as its canonical word order and it is also a wh-in-situ language, but scrambling of the object to presubject position (i.e., movement that results in OSV word order) is generally optional; however, in the context of negative questions with a NPI subject (e.g., amwuto “anyone”), (a) object wh-phrases must scramble on the wh-question reading and (b) the nonscrambled order has a yes/no-question reading. These properties of Korean wh-constructions with NPIs constitute POS problems for nonnatives whose L1 is English (as well as for native Korean-acquiring children). L1-English adult L2 learners (n = 15) and L1-English child L2 learners (n = 10), independently assessed for Korean proficiency, as well as L1-Korean children (n = 23) and L1-Korean adults (n = 15) completed an elicited-production task, an acceptability-judgment task, and an interpretation-verification task. The results show that (a) high-proficiency (adult and child) L2 learners performed like the native adult controls on all three tasks, thereby demonstrating L2 POS effects; and (b) adult and child L2 learners follow the same (inferred) route to convergence, a route differing from—yet subsuming—the L1-child route. Both sets of results lead us to conclude that, contra the FDH, the nature of language is fundamentally similar in natives and (adult or child) nonnatives.