The research I know shows that if you look at both languages of a bilingual child...
they start talking in at least one of the two languages within the normal age range.
It could happen that in the language in which they get the least input, so in which they have the least contact...
that they start speaking later than their monolingual peers. But that makes sense, because they hear less language input.
But it is not the case that bilingual children always start talking later than monolingual children.