One reason for casting doubt over the direct availability of Universal Grammar for adult second language accquisition is evidence that there is a critical period for language acquisition. After puberty, it seems, effortless and successful acquisition becomes less likely:

[…] language acquisition might be like other biological functions. The linguistic clumsiness of tourists and students might be the price we pay for the linguistic genius we displayed as babies, just as the decrepitude of age is the price we pay for the vigor of youth. (Pinker,1994: 296)

Evidence for a critical period comes from the unfortunate children, such as Genie (Curtiss, 1977, cited in Lightbown and Spada, 1999: 20) who were denied access to language input during the normal acquisitional period. Genie was 'rescued' from shocking deprivation at the age of 13 and despite the best efforts of linguists and psychologists to develop her language in a supportive environment, her language after five years showed little evidence of normal first language acquisition. Use of grammatical forms was inconsistent, there was much use of formulaic speech ('chunks'), and certain syntactic features always found in normal acquisition were missing (Lightbown and Spada, 1999: 20). In fact, the following examples of her speech resemble early interlanguage:

Studies of second language learners suggest a strong advantage for those learners starting before puberty at least in an environment where the second language is used on a daily basis. However, in standard instructed environments, young children appear to do significantly worse than either adults or adolescents (see Lightbown and Spada, 1999).

It seems, then, that children, either because they have access to Universal Grammar or for some other reason, are particularly well adapted to 'pick up' language without conscious awareness. This ability, however, appears to diminish in adults. This is not to suggest that effective second language learning in adulthood is impossible: far from it. Rather, adults are rarely able to acquire a second language to native speaker levels of competence. There are a number of case studies of adult second language learners who do (see Skehan, 1998: 218-22), but it seems that such adults require very powerful implicit induction skills and memory to pick up on the full range of structures which represent mature grammars.