My understanding is that you can obviously learn a language after a certain age, but that learning happens differently later in life than in early childhood

OP's article was about what it means to "know" a language, not what it means to be competent in a language, so of course my comment was about the effects of language on cognition. The only point my original comment made is that childhood language acquisition (no matter how many languages that may involve) and language learning later in life are qualitatively different. I'm not sure why that's controversial or harmful

That's all true, but at that point you're not using the same cognitive mechanism that you used in childhood to acquire a first language

It's risky to compare language acquisition to other kinds of knowledge or learning, because strictly speaking you don't "learn" your native language the same way you might learn a second language later in life.

From what we know about it, first language acquisition is automatic, begins in infancy, and occurs even with a paucity of stimulus. That is to say, your knowledge of your native language exceeds the sum of the language you've heard. Your capacity to acquire a language also diminishes over time. By the time you've reached adulthood, the critical period for acquiring a language has passed and you're stuck with what you've got*.

There's also a whole lot we don't know, such as why it's almost impossible to learn a second langauge with native-like proficiency later in life given first language acquisition is effortless. Second language learners can reach a point in learning where no amount of effort or immersion in the target culture will help fix the errors that remain in their understanding of the second language, and we don't know why this happens.

Language acquisition probably has more in common with learning to walk than it does with learning to ride a bike, that is to say it's innate and somehow you know when you're ready to do it, even before you're old enough to know how to not shit your pants. So, discussions of what it means to "know" a language need to differentiate between the processes of language acquisition and language learning, and be clear-eyed about how bad we are at the latter even though we're miraculously talented at the former.

*To clarify this point, because a lot of people took issue with it, I don't mean you can never learn another language, only that the process of learning a language in adulthood will be different to the process of acquiring a language (or multiple languages) in early childhood. This is an important consideration if you're going to talk about what it means to "know" a language

I heard he had trouble getting his music career moving. It was always four steps forward, four steps back with him