7 or 8 usually. Although some can talk earlier than that. For example, my daughter when she was 6:
Daughter: You know there’s a lot of people who are lonely?
Me: Yes darling
Daughter: Well what if all the lonely people got together and were friends with each other?
The common error I find in fiction is when a child who is way too young to have developed awareness notices that a parent is… [insert some subtle emotion or thought here]. Children younger than about 8 or 9 have very little awareness of Theory of Mind ie other people’s thoughts and feelings. They will for example, assume someone crying is sad and someone smiling is happy, so may become confused when a person cries with happiness. Another common error I find is a child having opinions about romantic relationships - generally you don’t see this until the tween years. I get fed up (as a reader) of every single child being a precocious genius.