Hi,
I would like to hear your opinion. I have lived in England for 7 years now, I moved here when I started to work. I am not bilingual I speak two languages, but it does not mean I am bilingual. I cannot think, reasoning or even dream in English. I have never ever received any education in English. I can use the language in my workplace, and I would say it is a lot easier as the time pass, but I never ever will become bilingual.
I have a son who is 3 and a half and I would say he is bilingual in English and my mother tounge, however he speaks another two languages. He is able to think, reason and also dream in English as well as my mother tounge. He just switch his brain when we go to the nursery and switch back when he comes home. His reading, maths, sicence, etc.. developes in the same speed in both languages. With the other two languages is completely different, he can speak that languages, making conversation, but he needs to make a lot of effort to be able to think or reason on these two languages (he is learning these two languages from our au-pair). Either he speaks four languages very well, I would not say he is multilingual. I think you can become bilingual if you receive the education (maths, reading, science, geography, biology, history, arts etc.) on those languages, becuase the education helps you to feel that language and culture closer to yourself. My son receive the English education in the English nurseries and schools, and I teach him at home to our culture and our education.
I think there is a big difference between able to speak two languages or become bilingual.