Final answer:
Babies can start to understand and distinguish between others' happy, sad, and neutral faces around 6-9 months of age. By this age, infants have developed the ability to recognize facial expressions, mimic emotions, and show preferences for certain emotions.
Step-by-step explanation:
Research suggests that babies can start to understand and distinguish between others' happy, sad, and neutral faces around the age of 6-9 months.
By this age, infants have developed the ability to recognize facial expressions and can mimic the emotions they see. They may also show preferences for certain emotions, such as smiling in response to a happy face.
Additionally, infants show a stronger recognition and attachment to familiar faces, such as their parents, compared to unfamiliar faces.