Native Spanish early and late acquirers of English as well as native Vietnamese early and child acquirers of English made grammaticality judgments of sentences in their second language. Native Spanish early acquirers were not distinguishable from native English speakers, whereas native Spanish late acquirers had difficulty with all aspects of the grammar tested except word order. Native Vietnamese early acquirers had difficulty with those aspects of English that differ markedly from Vietnamese. Native Vietnamese child acquirers had more generalized problems, similar to those of native Spanish late acquirers. Thus, native language appeared to make a difference 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 rapidly decoding surface form is offered as a possible explanation for both effects.