I give you two examples, me and my stepdaughter.
I grew up in the Netherlands, then when I was 14 I moved to the UK where my father had found a job in the oil industry. I am pretty comfortable with English, but still my Dutch is better. Could I call myself bilingual or not?
Another example is my daughter. She moved from Latin America to the UK (and later she moved to the Netherlands) when she was 6. Her mother speaks Spanish. From that moment she grew up with three languages that were constantly code switched: English, Dutch and Spanish. Now I could say she is trilingual even, but I notice she makes gender mistakes in Dutch. And sometimes I can even catch her making some small mistakes in Spanish. She is almost 'in between three languages'. She is fluent, fast and eloquent in three languages, but still not without errors. Dutch being the language that would her mother tongue I think.. but if I would call it her mother tongue, it is still not perfect.

