Age: 12-15 months

Q: Why does my toddler cry when he hears another baby crying?

Answer: Your child is starting to show early signs of empathy! Toddlers often imitate the distress of someone else in an effort to better understand what the other person may be feeling.

Source: https://www.nytimes.com/1989/03/28/science/researchers-trace-empathy-s-roots-to-infancy.html#:~:text=From%20a%20few%20months%20after,than%20a%20minute%20or%20two.