Five-month-old infants can tell the difference between people talk and monkey talk. Researchers showed infants pictures of people faces or of rhesus monkey faces at the same time they played sounds of either words in Japanese or monkey vocalizations. By measuring relative attention spans, the researchers inferred that the kids know which sounds should come from which creature.
We all know that cows go moo and cats go meow (at least in English). The question was when we discover that, and now we know it's by 5 months.