The relative conformity with which (typically developing) children attain adult grammatical competence—ultimate attainment—and the similarity in developmental paths along which they progress is remarkable (e.g., Ambridge & Lieven, 2011; Clark, 2003; Guasti, 2002; Synder, 2007). This achievement is, however, so ubiquitous and mundane that we seldom marvel at it. Of course, monolingual adult grammars may also differ from one another, especially for some domains of grammar (e.g., Dąbrowska, 1997, 2012), but such variability pales in comparison to the variation in adult non-native second language (L2) grammars. Indeed, the path and outcomes of L2 acquisition can be highly variable from one individual to another, even under seemingly comparable contexts. Individual and group-level factors in adulthood that either do not apply or apply with much less consequence in young childhood conspire to explain at least some of the gamut of L2 variability.