Not so, perhaps for the majority of people in the world. "Shifting L1" is a natural and common phenomenon. It means that your primary language shifts from one to another over time. For example, my wife was monolingual in Taiwanese Hokkien before she entered kindergarten. Her parents only spoke Taiwanese Hokkien at home. But after she started schooling, she became proficient in Mandarin. It was back then when the use of so-called "dialects" were discouraged in schools. She always chatted with her friends in Mandarin. So, soon, Mandarin became her dominant language. Now, although she can understand Taiwanese Hokkien with no difficulty, she's not very confident in it either; except for some limited purposes, such as chatting with elder members of the clan, or shopping in traditional markets.
The majority of people in the world live in multilingual societies. In such societies, the language you speak at home may be different from the dominant language of the particular location they live in, which may again be different from the official language or national lingua franca. For example, Juan, a Filipino adult, comes from an Ilonggo family. His parents spoke Ilonggo at home when he was a young child, so you may say that his "mother tongue" is Ilonggo, which is a regional lingua franca in some provinces in the central Philippines. However, his family lived in Manila, so none of his classmates spoke Ilonggo. They spoke "Taglish" (A mixture of English and Tagalog, the vernacular of middle-class manilenos). Soon, Taglish became his dominant language. But as the school he went to was a prestigious private school where people mostly spoke English only, so by the time he graduated from college, English became his dominant language. He can still use Tagalog to conduct casual conversations, but he can't really talk about all topics in straight Tagalog. Moreover, he has forgotten most of his Ilonggo, except some basic expressions. So does it still make sense to say that his first language is Ilonggo? And does it make sense to say that he's a non-native speaker of English, when it is the language he is most proficient in, even though he obviously does not sound like an American when he speaks it (in fact, he has a quite strong Philippine accent)?
