Raphtalia may not be the leading character in The Rising of the Shield Hero but she certainly steals the show with her cute and kind demeanor. She's easily a fan-favorite among the many characters in the isekai anime, and for good reason -- she's a half-human, half-Tanuki hybrid who accompanies Naofumi throughout his journeys and, though she was initially purchased as a slave, she has been able to grow and develop as a person. But fans are often confused by her age, and it makes total sense as to why.
When Naofumi first meets Raphtalia, she has the appearance of a young child, and her behavior falls right into line with that. According to her own story, Raphtalia was raised in a small coastal village until the age of 10, when the first calamity struck and she lost her parents to the disaster, eventually becoming a slave. She was in the possession of Idol Rabier when Naofumi first crosses her path and changes her life forever.
Soon after joining Naofumi, Raphtalia's physical body begins to mature rather rapidly, quickly becoming that of a young woman. Yet her personality doesn't seem to mature as fast as she maintains a somewhat immature attitude. It's very quickly revealed in The Rising of the Shield Hero that demi-humans such as Raphtalia mature in direct relation to their progression of levels, which is why Raphtalia matures so quickly -- she levels up a lot faster once Naofumi allows her to experience a modicum of freedom.
But her age in the anime in its current state is still not totally clear. In the light novel version of the story, Raphtalia is supposedly 17 years old, but even that isn't confirmed beyond doubt. Many Shield Hero fans seem confused because while she has the body and level of a 17-year-old, she still seems to be a 10-year-old mentally.
In the Shield Hero anime, there is no canon reveal of Raphtalia's biological age yet. Because she increases in both physical and mental stamina every time she levels up, one could argue that she ages every time she moves up a level. In addition, Raphtalia is very traumatized after watching her parents die and her entire village destroyed, which definitely mentally matures a person in one way or another. While Raphtalia still exudes that inner 10-year-old from time to time, her experience both prior to and with Naofumi has aged her both physically and mentally.
Raphtalia is one of those characters who will remain an enigma, and her lack of a coherent, biological age doesn't help matters. As the creator, keeping her age ambiguous can help with the story down the line; for example, deciding to form a romantic bond between her and Naofumi would require her to be of age for it to be deemed appropriate.
So on some level, as the anime certainly hints at that happening in the future, Raphtalia would have to be older than 10. While there may be no canon age for Raphtalia, her immature attitude will always keep her seeming lively, youthful and a little bit dramatic.