Let's consider vision as an example. This happens because the brain was not stimulated during the critical period of visual development. In contrast, if an adult eye is deprived of similar input after the critical period of ocular development, by being patched for a period of time, for example, there will be no lasting deficit. Even though other abilities are not dictated strictly by the critical period phenomenon, an area like language development does have peaks and plateaus. Between the ages of 8 months and 3 years a child's language comprehension and strategies become more sophisticated.
Children are best able to learn a new language when they're young.
There's no sound data to support the belief that speaking two languages at home will impair a child's language skills. This area of research is not specific, since we do not have a clear understanding of whether emotional deprivation actually causes changes in the brain or changes in behavior. Keep in mind that they were dealing with extreme deprivation and physical or psychological neglect. Not every child who experiences neglect or bad experiences reacts in the same way. Genes matter, and experience matters.