One of the most common myths about bilingualism is that bilingual children “learn to speak later” than monolinguals. I’ve heard it, read it, said it, but strangely, I haven’t experienced it at all with my three bilingual kids. Now, as a researcher, I know that three kids does not a “reliable statistical study” make… but certainly anecdotally, I have not found this to be the case. In addition, although I’ve heard this myth a lot, and read it in several popular books, I’ve never seen convincing research on the topic. Well, convincing research is on its way, and it comes down firmly on the side of “myth” (for now, anyway!).
This article does a good job of explaining the evidence, and refuting the claim of later language for bilinguals. As an aside, Annick De Houwer’s books are a great reference for parents of young simultaneous bilinguals, although they are definitely “research reading” and not “easy” reading.

From the Linguistics Research Digest Blog:

Do children hearing two languages acquire language at a slower rate?