An interesting question.
My first instinctive answer – anyone from an English speaking country who learns from birth from a parent – falls apart now I stop to consider. A person could be raised in a non-English speaking country by native-English speaking parents and that would, in my opinion, also make them a native speaker.
Tricky. I could say they need to learn it from birth and have native English speaking parents but on the other hand, there are plenty of second generation British people, whose parents may not actually have good English, but who themselves I would consider native speakers as they have grown up in a native English environment, and their English is perfectly ‘native’.
So I guess I would say there needs to be at least one of two conditions:
To grow up exposed to the language of a native English speaking country and/or
To have native English speaking parents.
But again, on consideration there are problems with this. Someone whose parents are native speakers of a different language to the country they are growing up in, quite often are not completely fluent in their parents’ original language. (Again with the example of second generation immigrants in mind).
Also, some countries are officially English speaking, but it is not the same as what I would call a native version of the language. Again, an example, I know a lot of people from Nigeria, whose official language is English and have learnt it from virtual babyhood along with an African language, and have done all their schooling in English. However, they are not native speakers and would not describe themselves as such once they have encountered actual native speakers. One said it came as a terrible shock to come to Britain and find a terrible language barrier after speaking what he thought was British English for nearly 50 years. He was unable to understand most people, and most people were unable to understand him.
So, what does everyone else think? It is just people from Britain, USA, Australia and NZ who are the native speakers or do other versions of English also count as ‘native’? If I’m being really cheeky, perhaps only British people speak native English – as even the USA has left the British version behind, and it was our language originally?