Why do adult language learners typically never achieve the same level of language mastery as native speakers? We examine how prior knowledge and experience might influence the size of the linguistic units employed in learning, and as a consequence, what gets learned. We hypothesize that adult learners tend to learn from more segmented representations, and that these can hinder learning about the relations between segments (e.g. between articles and nouns). In particular, we focus on the acquisition of grammatical gender, an aspect of language later learners show difficulty with compared to native speakers. In a study of adults, we show that participants are better at learning grammatical gender in an artificial language when they are exposed first to article-noun sequences and then to noun-labels as compared to learners that start out with noun-labels and then hear article-noun sequences. This striking finding can be explained by a simple blocking effect. We discuss how the units children and adults learn from impact language learning.