@BCS: Mental age is not merely a factor of memory. The brain continues to develop as one grows as well as different chemicals and hormones influencing emotions and behavior, thus, an adult suddenly placed in a child's body would likely behave much more like a child than they used to.
They're being ambiguous about who or what kind of person this was before they got sent (physically/gender wise) but I think that works
All we need to know is mentally they're 19, but she's been sent in as her character instead.
Plus its really cute to see this instead of /another/ douchey blank slate isekai dude