You could also add a fifth area of speech and language, in which a person is able to use one or both languages for reasoning, to your definition of bilingualism. A person may speak only on language but have listening comprehension in two languages. Another may listen and speak in two languages but reading and writing ability in only one. The term balanced bilingualism is used to describe individuals who possess about the same fluency in two languages, while semilingualism refers to those who have deficiencies in both languages compared with monolinguals. These deficiencies could be in a reduced vocabulary, incorrect grammatical patterns, difficulty thinking or expressing emotions in one of the languages, etc.
Few people are truly balanced bilinguals in both languages in all situations.
One language is usually dominant. This dominance may be different for listening and speaking or for reading and writing and usually changes over time. At any rate, it's most useful to define for yourself what type of bilingualism is important and necessary in your family, within your community and culture, in order to pan a strategy for raising kids with the ability to use more than one language. Most of us are able to learn a second language at any time in our lives. And, thought no critical age for bilingual language development has been found, kids do tend to develop more native-like pronunciation when bilingualism begins before adolescence.