I think the better question is, when does being able to think and do math quickly in a new language disappear? Lots of theories in this thread, but pretty much

I think the better question is, when does being able to think and do math quickly in a new language disappear? Lots of theories in this thread, but pretty much everyone has a mother tongue in which they think and count. There might be tons of outliers, I'm sure, but for the vast majority of multilingual speakers, your native language is the one you go back to in your head when you're trying to calculate the tip or count pennies. Yes, as odd as it seems, we use language when doing math. Are you multilingual? Have someone give you a basic math problem verbally, and then give them the answer. Which language did you use? Most everyone will switch to their native language, and then switch back. So again, when does this ability disappear? I moved to Spain when I was 30yo and learned how to live and work in Spanish, but I'll never be truly fluent. English just is how my mind works, and Spanish is really just a bunch of synonyms in my brain. It's all in one giant bucket, and just like I don't swear in front of small children and use simpler versions of words to talk to them, and use strict noun-verb-object sentences (with no phrasal verbs!!) when speaking "international English" to someone for whom English is a second language, I speak Spanish words in a certain order for Spanish speakers. This is not how truly multilingual people think. What also is interesting is how growing up in a multilingual environment changes how fast you can context switch. Someone who grew up in say rural France and had no exposure to English, might learn it amazingly well, but if they're in mixed company, you can watch their heads explode trying to swap languages. I love the blank stare they'll give you as they change brains, recall what you just said, and then are able to respond. Then of course you have the Spanglish teens of LA who don't even notice they're mixing languages in the same sentence. I think all this is to say, it's a tad more complicated than just "when" and "if".