When does a thing become a person, in any language. When is it correct grammar to refer to a thing as a person?
It depends on the language, really.
Many languages have a distinction between "animate" and "inanimate" baked into the grammar, but they don't all draw the distinction in the same place. For a few examples:
- In older Swahili, gender #1 (m-) is used for most people. Animals use gender #9 (N-), and "irregular" people (defined by a specific characteristic, including disabilities like blindness, but also height) use gender #7 (ki-).
- In modern Swahili, this distinction is vanishing, with all humans and animals shifting into gender #1.
- In Ancient Greek, all words for adult humans are grammatically animate, but small children are grammatically inanimate.
- In fact, the word for "living being" in general (ζῷον zôion) is grammatically inanimate, and includes not just humans and animals but also paintings and sculptures.
- In Classical Latin, all words for humans and animals are grammatically animate, but so are the majority of all other nouns. The neuter gender (descended from the PIE inanimate) died out completely in later Latin, which is why the Romance languages don't have it.
And the lines aren't necessarily sharp and strict, either: look at English, where it's valid to call an animal or infant "it", but equally valid to call it "he" (or "she"), and there's a convention of using "she" for ships and other large vehicles.