This confused me too, for a bit, since I always thought she was 14, max. But with this/these recent revelations, as well as something from the Halloween event a while back, it makes sense that she is at least 16.

First off, in Japan, 16 is a legal age, so nothing is "wrong" per se. But the reason this age makes a but more sense is taking into account her in game model. She uses the female midlanders skeletons model. This is proven by not only her in game emotes but her glamour from the Halloween event in 2019.

In game, the older teens use the adult models, so this fits with Ryne as well. The main reason why I think so many people were "confused" about her age was because of her height, as I think she is shorter than the shortest female midlanders; if someone can confirm this it would be nice. But anyways, that leads me to believe, she's just naturally short. Which, given the "stasis" her body had been going through until recently, it makes sense why her physical body would be "underdeveloped" compared to other individuals.