In the first season of the show, Midoryia explains how the quirks popped up and all, he says that the first one was a child in Qing Qing city, in china. This is where it takes a turn. Everyone so far that you see in the show, or read about in the manga, (for those who aren't explained, it's assumed they manifest at age four). So I was wondering why the first person who manifested their quirk was a newborn. I don't understand.
This might be linked back to the quirk singularity theory.
In the quirk singularity theory, it was explained that, as the generations pass, quirks become stronger and harder to control. This technically also means that, as the generations pass, children have a higher chance of having stronger quirks and also leads to the percentage of quirkless people to lessen, meaning that it is more likely for a child or, in this case, a newborn to have a quirk as compared to people from other generations.
Additionally, it was mentioned that people start to get quirks before/when they turn 4, meaning that, while it is more common for a quirk to appear when someone is 4 years old, it is not impossible for a child to get a quirk before they turn 4: similar to how Present Mic got his quirk when he was born and in turn made the doctor and his parents' ears bleed when he cried.