It contains a nice little encapsulation of an idea that is often treated as some kind of universal truth:
When we say language acquisition, [we] mean either the first language acquisition or the second language acquisition. In this domain, we should distinguish between the first language and the second language. The first language which acquires by a very young child (mother tongue), the second language which acquires by older learners, and it includes any language which learners acquire except their first language (mother tongue).It's this first language (1L) / second language (2L) dichotomy that I'm skeptical of. The idea generally seems to be that when you're a kid, some language becomes your first and only native language. But I don't think that's always the case.
I'll use the example of my own daughter to disprove such an expansive rule. She has spoken Chinese, English, and Japanese for her entire speaking life. When she started speaking, we were living in China, and her strongest language was Chinese, followed by Japanese and then by English. We then moved to the States, where the order became English, Japanese, Chinese. Now we're in Japan and the order is Japanese, English, Chinese. It'd be very hard to label one of these as her "first" language, and saying that she has three "first" languages doesn't make a lot of sense either.
I think the more appropriate distinction here would be between "native" and "non-native" languages. Native languages seemed to be defined by two characteristics: (1) you learn them when you're small when your brain is particularly wired for certain language-learning activities (like accent acquisition) and (2) you learn them initially by pure exposure.
The fact that the first characteristic above is limited to very small children is really the main difference. You can always try to learn a language (very inefficiently) through pure exposure, but your brain will never be set up quite the same way in terms of language learning as when you're a very small child.