A foreign language is a language that is not an official language of, nor typically spoken in, a given country, and that native speakers from that country must usually acquire through conscious learning - be this through language lessons at school, self-teaching or attendance of language courses, for example. A foreign language may be learnt as a second language, but there is a distinction between the terms, as a second language may be used to describe a language that plays a significant role in the region where the speaker lives, whether for communication, education or government, and therefore a second language is not always a foreign language.
Children who learn more than one language from birth or from a very young age are considered bilingual or multilingual. These children can be said to have two, three or more mother tongues, and so again these languages would not be considered foreign to these children, even if one language is a foreign language for the vast majority of people in the child's birth country. For example, a child learning English from his English father and Irish at school in Ireland can speak both English and Irish, but neither is a foreign language to them. This is common in countries such as India, South Africa, or Canada due to these countries having multiple official languages.
In general, it is believed that children have advantage to learning a foreign language over adults. However, studies have shown that pre-existing knowledge of language and grammar rules, and a superior ability to memorise vocabulary may benefit adults when learning foreign languages.