There are strong indications that a native language will stabilize around age 12. If a child is removed from the native language community before that age, he or she can forget this language to a large extent, or even entirely. The same is true for children who are born to families that speak a different language than the one used in their surroundings (such children are called ‘Heritage speakers’).
Many parents do not believe this and feel that the language knowledge a child has at age six, eight or ten must be strong enough to resist erosion. In particular it is often thought that a strong and consistent policy of using the native or heritage language in the home should prevent language loss. While this certainly helps (for more information see here), it is no guarantee and there are no absolute safeguards.
It is more common for such children to use their native language in ways that are typical of second or foreign language learners than in ways that are characteristic of native speakers: They will often have a strong foreign accent and not fully master some or many of the grammatical rules of their language.