I feel like that leaves a little weird wiggle room though.

Let’s say you’re born in a Spanish speaking country, maybe Mexico, for the first few years of life you grow up surrounded by Spanish speakers, your first words are in Spanish, you only know Spanish, everyone you know only speaks Spanish.

Then when you’re about 3 years old, before you’re even forming really solid, permanent memories, you go to live in the US, you’re surrounded by English speakers, almost everyone around you stops speaking Spanish regularly and switches to English, your English vocabulary quickly catches up to or maybe even surpasses your Spanish ability. Your first real memories are of people speaking English, and you spend the rest of your life primarily speaking English. You still speak Spanish though, you keep up with your education in that language and can speak both fluently.

I think there’s a valid argument that both could be considered your native language, even if Spanish was your first language, you’ve still grown up speaking both.