Idea for a Generic Medieval Fantasy Setting: The characters refer to their nameday as an apparent stand-in for birthdays, celebrating it annually according to their respective preferences and perhaps family customs, as one does. People talk about things that happened before someone's time as having gone down "before you were named", someone grievously insults an opponent on the battlefield by going "your mother should never have named you." So with the way naming is always talked about, as a reader you start to somewhat assume from context clues that these people have some sort of a taboo about the word "birth" or something, and naming is used as some sort of an euphenism to avoid naming the process in which people come into the world.
Then somewhere halfway through the story it turns out that in this setting, people aren't named immediately after being born. This is a semi-realistic-gritty fantasy setting, after all. Due to the somewhat high infant mortality, to at least somewhat soften the blow of potentially losing a child, babies just aren't named before the parents are pretty confident that the kid is going to survive. The naming ceremony is where a baby is officially aknowledged as an entire individual, a member of the family and a legally existing person, instead of just a gurgling extension of the mother who may or may not disappear from this world. And that timespan between birth and being named is - depending on the situation and the family - somewhere between 1-4 years.
And suddenly the whole bunch of annoyingly-too-mature teenagers and other weird remarks about age start making sense in hindsight. The heroine protagonist who celebrated her 16th nameday at the start of the story is actually 19 years old. The wild difference in maturity between two characters who were both named the same year wasn't just a difference in backgrounds, The Rich Idiot isn't just rosy-cheeked and naive due to being sheltered growing up, but actually literally years younger than a peasant "of the same age". A character who's sickly and was frequently remarked to look much older than their years hasn't just been harrowed by their illness, but was not named before the age of seven because their parents didn't think they'd survive.
