English be and have stranding differ under vP ellipsis: *John slept and Mary was too vs. Peter saw your parents last week, but he hasn't since. Rouveret (2006) claims that -ing has an interpretable progressive feature that cannot be elided (unless recoverable), whereas -en has an uninterpretable perfective feature which deletes freely. In Arabic, such contrasts do not arise. According to the Interpretability Hypothesis, uninterpretable features not realized in the L1 are not fully available to post-childhood L2 learners. Therefore, Arabic speakers who learn English beyond childhood are predicted to have difficulty distinguishing be and have stranding, unlike childhood starters. The present study compares 50 (Saudi) Arabic EFL elementary school (child) starters, 82 middle school (teen) starters, all now in their 20s, and 11 English controls. Results from a bimodal timed Acceptability Judgment Task show that even at advanced stages of acquisition, child and teen starters incorrectly reject Peter saw your parents last week, but he hasn't since, suggesting that for them -en has an interpretable perfective feature. While the Interpretability Hypothesis predicts the teen group's performance, the identical performance of the child starters is unexpected. The implications of this for a Critical Period for L2 acquisition are considered.