Tsimpli and Dimitrakopoulou (2007) observed that uninterpretable features are unavailable in second language (L2) acquisition after the critical period. In this paper, we verify this claim by providing evidence from Persian speaking learners of English as an L2 on the status of resumptive pronouns (RPs) as uniterpretable features. Unlike English which does not allow RPs, Persian shows various behaviors across different relative clauses (RCs). In Persian, RP is ungrammatical in subject, optional in object, and required in object-of-preposition RCs. To examine the status of RPs in these learners' interlanguage, a grammaticality judgment test and a translation test were developed and administered to 111 adult Persian learners of English at four proficiency levels and 18 English native speakers. Repeated measures ANOVA results, tracing the effect of proficiency on different RC types, suggest that as their proficiency improves, learners become more native-like in rejecting RPs in English. However, in comparison with the native speakers, even advanced learners show marked performance deficits notably in object and object-of-preposition RCs. These results are in line with the predictions of the Interpretability Hypothesis proposed by Tsimpli and Dimitrakopoulou. The findings also provide some implications for the age-related issue in L2 teaching.