When parents speak more than one language it can be difficult to choose whether to introduce both or all of the languages from birth, or to rather wait until the child’s skills in one language have become established before introducing the second (or additional) language(s).
Simultaneous bilingualism vs Sequential bilingualism
Simultaneous bilingualism is when bilingualism is achieved via acquiring a first and second language concurrently.
Sequential bilingualism is when bilingualism is achieved via learning a second language later than the first language. Some also call it subsequent bilingualism.
But what means “later”? A month, a year later than the first language? Usually, we consider a sequential bilingual someone who learnt the second language after the age of three.
If the second language is acquired before age of three, and in a more natural, not guided way, we talk about simultaneous bilingualism.
Both labels only indicate the order in which languages are learnt!
They don’t indicate whether a child becomes a balanced bilingual or not!
This depends on many factors that are not intrinsically related to whether the child acquires both languages from birth or not.