While there are some natural advantages to being a child (see the next point), there is no cut off point at which you become less capable of learning a foreign language. The idea that it’s best to do it when you are young is generally untrue, and you will probably find a significant portion of learners who did not achieve fluency in a second language until after high school.
We all can learn like a child does
Children learn to speak fluently by having years of nothing but constant native-speaker input and constant prompts to communicate. For about 5 of these years, the child constantly has a person nearby whose primary role is their development, and they are constantly engaged in level-appropriate communication.
In addition, the child is hugely motivated by the necessity of daily life and the interest they take in relatively simple things such as children’s cartoons and stories. Despite this, after 10 years, the child still not capable of speaking as fluently as an adult and handling all of the communicative tasks an adult needs to perform to live. Even the most mediocre language learner beats this record. There is nothing magical to being a child.
Adult learners, on the other hand, can bring their intellect and knowledge to bear to progress far faster than a child at the early stages of learning. Adult learners have the advantage of understanding what grammar is and how it can be applied, the willingness to spend time actively learning new vocabulary, as well as the concentration that lets them sit and study for hours at a time.
In later stages, the child can outpace the adult on the back of the enormous amounts of native communication they engage in. This is a natural result of the different circumstances surrounding these two kinds of people. Adults can and do achieve native-like fluency, and they do it with a smaller amount of language hours than the child. This is not to say that you should not use a lot of native input. It is essential. This post serves to dispel the myth that adults ought to learn purely through native input, as in the case of children. Adults and children learn differently, and their methods reflect that.