The meaning is unknown because the characters in Genshin Impact are not told their age at all. In fact, the official site that stores Wikis regarding these characters also does not mention their age.
Let’s Talk About Fischl Age
If you look at this website, it can be seen that Fischl’s birthday is on 27 May. Unfortunately, as for the year of birth, she herself was not made public at all. However, we can actually find out the age of the characters in the game easily. You see, the characters in Genshin Impact have various appearances, which makes us immediately know how old they are.
As for Fischl herself, she is a character who works as an investigator for the Mondstadt Adventurer’s Guild. In this job, the Guild does not provide information on the minimum age that can work here. So, you could say Fischl is not 20 years old and above.
Moreover, Fischl also has a Chuunibyou trait, which we know is a delusion of a teenager. So, we find out that Fischl has less than 20 years of age because she is currently someone who is still a teenager.
So, with all the conclusions and data obtained, it can be said that Fischl is still 16 years old. Considering Fischl still has a young face and makes friends with people under 20, this makes much sense. What’s more, Fischl also has the voice of a teenage girl, which makes her is illegal (global age). In contrast to other female characters, such as Lisa and Jean, whose voices and appearances really look like they are 20 and over.