Scientific American

We wanted to know whether and when babies might discover the importance of a talker’s mouth. So, in one study in my lab, we showed videos of talking faces to babies of different ages and tracked their attention by using an eye-tracking device. We discovered that babies begin lip-reading at around 8 months of age. Crucially, the onset of lip-reading at this age corresponds with the onset of canonical babbling, suggesting that babies begin lip-reading because they become interested in speech and language. By lip-reading, babies now gain access to visual speech cues which, as Janet Werker and her colleagues at the University of British Columbia have shown, are clearly perceptible to them. So, the lip-reading now enables babies to see the visible speech cues that they need to figure out which face goes with which voice. Of course, babies cannot access visible speech cues if others are wearing masks.

https://www.scientificamerican.com/article/masks-can-be-detrimental-to-babies-speech-and-language-development/