Some people believe you cannot be a "real" bilingual if you have not acquired your two languages in infancy or at least as a young child. It's true that there is something magical about a toddler who speaks one language to her father and another to her mother. But in fact, one can become bilingual at any time during one's life - as a child, as an adolescent, or as an adult.
Children who acquire two languages simultaneously have been the object of many studies and books. Their parents often adopt an approach that allows them to receive two language inputs but they are in fact rarer than children who acquire their languages successively.
The majority of child bilinguals start monolingually. They first acquire a home language and then, usually when they start going to school, they learn a second language, most often the majority language. Literally millions of children throughout the world have become bilingual in this manner. Then, depending on the country they live in, they may start learning a third (and even a fourth) language as a school subject.
Older children and young adolescents may also become bilingual. This will happen when they move to another linguistic region or to another country and start to be schooled in their new language. (This was my case when I was eight). Writer Eva Hoffman relates vividly in Lost in Translation how at age 13 she arrived in Canada from Poland and was immersed in a new language and culture.
Can one become bilingual later on? There is no upper age limit for acquiring a new language and then continuing one's life with two or more languages. Nor is there any limit in the fluency that one can attain in the new language with the exception of pronunciation skills. Acclaimed novelist Agota Kristoff fled Hungary at age 21 during the events of 1956 and came to the French-speaking part of Switzerland. She acquired French from scratch and many years later published her first novel.... in French!
The main factor that leads to the acquisition and development of a language is the need for that language - the need to interact with others, to study or work, to take part in social activities and so on. If the need for a language is present, then language acquisition will usually take place. This is true of children as it is of adults.
Other factors must also be present: enough language input and use; the help of family, friends, colleagues, and the community in general; formal language learning for some; and positive attitudes towards the language and culture in question, as well as towards bilingualism.
Some might ask: Isn't it better to start becoming bilingual as early as possible? This really depends. For example, as concerns need, a second language may not be needed early in life, hence the fact that bilingualism may develop later on only. If the need for a language is not present, or no longer present, then the language may not be acquired or may become dormant.
There is also the question of language fluency. Some researchers have downplayed the "earlier you start the more fluent you become" argument. It is true that for native-like pronunciation, the "sensitive period" range is shorter but even then I have known bilinguals who have acquired their second language at age 15 and have no accent in it. As for other skills, the window is not as clearly marked and acquisition can take place at any time.
Admittedly, some underlying listening and speaking mechanisms may not be the same as those of early bilinguals but this does not usually impede communication.
"Life as a bilingual" posts by content area: http://www.francoisgrosjean.ch/blog_en.html
François Grosjean's website: www.francoisgrosjean.ch