Admiral James Kirk wrote:
It's not essential to the story that she be the same age. But the character's novelty would be lost if she were an older teen. Instead of Batman in Robin's body, she becomes Buffy with a foul mouth.
I see what you're saying. The novelty in her character was the fact that she was so young but could really kick anyone's ass. So if she is a teen doing that the novelty is gone.