How would you define a native english speaker?

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Hi!!! I was born in the States and raised there as a kid. We moved to Mexico when I was 13 years old, and quickly learned spanish as I had heard it at home almost all my life. I´m 38 years old now and i went on a job interview where i was asked if i was a native english speaker,,,I answered yes, i always considered myself one. Then later on someone explained to me that your native language is the one you feel more comfortable speaking. Obviously now it´s spanish since i´ve been speaking it over 25 years, so now i wonder.....Is your native language the one you learned first and spoke since the day you were born? Or the one you speak better? Or the one you feel more comfortable speaking???