Conclusion Wild and isolated children and the critical age issue for language learning
Most significant for the Critical Age hypothesis is the fact that Isabelle and Helen started to learn language at an early age following a relatively short period of deprivation: language exposure and teaching began for Isabelle at 6 years old and for Helen at 7 years. On the other hand, Victor and Genie, who were almost twice as old as these girls when discovered, did not learn as much language as Helen and Isabelle did, nor did Chelsea. This could be, as some theorists, such as Lenneberg (1967), have suggested, due to the biological maturation of the brain.
In any case, to rely only on the cases of Victor, Genie, and Chelsea to make a case for critical age is surely not warranted. There are too many unknown factors and the data, particularly for Genie and Chelsea, are not clear. Certainly, the critical age, if there is one, could not be much younger than 6 or 7 years since that would exclude Isabelle and Helen, who in fact did learn language after that age. Clearly, the ideal experimental situation for studying the problem of a critical age for first-language learning has not yet presented itself. Let us hope for the sake of some poor child that it never does.