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.
And, as Mark Millar has said that Kick-Ass 3 will be moving the story ahead 10 years, we'll be getting a "Buffy with a foul mouth" version of the character eventually anyway. So why rush things?
Of course, one good thing about that is that it will give Chloe and Mindy a chance to catch up to each other again before the final movie.

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