Generally, I refer to people as bilingual if they grew up in a household where two languages were used equally so they absorbed the languages to the same native level. I work with a Dutch lady who is married to an Icelandic man and they moved together to the UK four years ago, before having a baby. Their son, now aged 3, speaks fluent Dutch, Icelandic and English. At home, they speak only Dutch and Icelandic but their son is exposed to English in every other aspect of his life so speaks it at native level. He is, in effect, trilingual.
An alternative method of becoming bilingual is to live in a country from a young age even if the native language of that country is not the one spoken at home. A British friend of mine moved to France about ten years ago, with her husband and two small children. My friend spoke fairly good French before they moved there but her husband spoke none. Needless to say, both she and her husband now speak very good French but I wouldn't call them bilingual. Their children, who were aged 1 and 3 when they moved to France, are (in my opinion) bilingual. They attended only French schools, all their friends are French, and French people can't tell that the kids weren't born in France to French parents.