Native Spanish early and late acquirers of English as well as native Vietnamese early andchild acquirers of English made grammaticality judgments of sentences in their second language.Native Spanish early acquirers were not distinguishable from native English speakers, whereasnative Spanish late acquirers had difficulty with all aspects of the grammar tested except wordorder. Native Vietnamese early acquirers had difficulty with those aspects of English that differmarkedly from Vietnamese. Native Vietnamese child acquirers had more generalized problems,similar to those of native Spanish late acquirers. Thus, native language appeared to make adifference for early acquirers, whereas a later age of acquisition caused a more general problem.A processing-based model focusing on the difficulty non-native language learners have in rapidlydecoding surface form is offered as a possible explanation for both effects.