The only place I’ve read or heard this is on advertisements for commercial language courses on tape or whatever they’re using these days. However, frequently and consistently this notion is ascribed to those advocating or presenting communciative or comprehensible input methods and techniques. Since none of the SLA material I’ve read makes that statement, I have been puzzling over where this notion that they do comes from.
With no evidence (that doesn’t seem to stop anyone else from making claims), I’ll say there are two, the first stemming from the second I present here:
Teachers coming back from workshops, in-services, and so on with only partially digested notions of communicative methods.
The second source is the LAD hypothesis: there is a hard-wired Language Acquisition Device in the brain, placed there by evolutionary processes, that allows us, nay, forces us, to learn language. And, here’s the key, this LAD doesn’t shut off just b/c a person has learned his first language. That does not mean that learning a second language in adulthood is just like learning one’s first language in infancy.
So that may be the source of the confusion: people can still learn a language beyond their first, in adulthood, b/c the LAD doesn’t shut off. Any dolt can surmise that conditions are different but the point is that, given comprehensible input, the brain, the LAD, will continue to act upon material in the TL that is comprehensible, although not “just like” an infant’s brain will.
Chomsky is associated with the LAD concept and Krashen credits him for that as tprs’s Blaine Ray credits Krashen for his underlying approach to fl learning.