Native speaker is often supplemented by detailing nondevelopmental characteristics that they share. Stern (1983) claims that native speakers have (a) a subconscious knowledge of rules, (b) an intuitive grasp of meanings, (c) the ability to communicate within social settings, (d) a range of language skills, and (e) creativity of language use.
The Encyclopedic Dictionary of Applied Linguistics (Johnson & Johnson, 1998) adds (f) identification with a language community. Davies (1996) adds (g) the ability to produce fluent discourse, (h) knowledge of differences between their own speech and that of the “standard” form of the language, and (i) the ability to “interpret and translate into the L1 of which she or he is a native speaker.”
These characteristics are variable and not a necessary part of the definition of native speaker; the lack of any of the would not disqualify a person from being a native speaker.
The indisputable element in the definition of native speaker is that a person is a native speaker of the language learnt first; the other characteristics are incidental, describing how well an individual uses the language. Someone who did not learn a language in a childhood cannot be a native speaker of the language. Later-learnt languages can never be native languages, by definition. Children who learn two languages simultaneously from birth have two L1s (Davies, 1991).
Asserting that “adults usually fail to become native speaker” (Felix, 1987, p. 140) is like saying that ducks fail to become swans: Adults could never become native speakers without being reborn.
L2 learning may produce an L2 user who is like a native speaker in possessing some of the nine aspects of proficiency detailed above to a high degree but who cannot meet the biodevelopmental definition. The variable aspects of proficiency (Davies, 1996) or expertise (Rampton, 1990) relate to a separate issue of quality rather than being defining characteristics of the native speaker (Ballmer, 1981).
Cook, Vivian. (1999). Going beyond the native speaker in language teaching. TESOL Quarterly, 33 (2), 185-209.