How old should a baby start learning a second language to be the same as his mother tongue?

For more than half a century, researchers have constantly verified and questioned this hypothesis from various angles, but so far no definite conclusion has been reached, but generally speaking, the younger the age, the easier it is to learn a language; The better the language environment and the richer the language information, the easier it is to learn a language. At the age of three, your ability to learn a language may not be as good as that at the age of one. At the age of seven, your ability is not as good as that at the age of three. Twelve years old, your creativity is low ... and so on. ? Therefore, the ideal situation is, of course, to immerse yourself in a multilingual environment from infancy, so that the brain in the high-speed learning period can absorb language information like a sponge, thus becoming a bilingual or multilingual at the lowest learning cost. ? For example, an intern who goes against the sky starts to learn a second foreign language in the fifth grade of primary school 1 1. In fact, it is already late, and it will go through a very difficult learning process. However, this child (I don't know if it really exists) has one advantage. His family immigrated to the United States and entered a local primary school. He has a good pure English environment. Immersing himself in the target language environment for a long time will certainly help him learn the language quickly. After three years in the United States, children's daily conversations should be no problem. To be the captain of the debate team, I can only say that he has a great talent for languages.