Three well-known cases of extreme linguistic isolation during childhood and two recent cases from the neurological literature involving left-hemispherectomy in children are examined. In all five situations, subjects underwent delayed L1 acquisition (with L1 onset ranging from 5 to 31 years). “End-state” utterances provided in published reports are analyzed for evidence concerning subjects’ control of the Head Position, Null Subject, and Wh parameters. In addition, the early phrasal development of a subset of the five subjects is investigated in terms of the asymmetric Merge operation. Findings concerning ultimate attainment indicate that the younger cases set parameters more successfully, and that performance declines markedly with increasing age, while results regarding early multiword utterances suggest that these are strikingly “normal” as long as delayed onset of L1 occurs within, and right up to, the critical period boundary. This pattern, it is argued, is consistent with the notion that pre- and post-critical period first language learning involve qualitatively different processes.