@ Negima takes place during the timeskip right before Kei and Naru's wedding. So she would actually be about Sarah's age.
IIRC Sarah was 14 at the Epilogue. The only ways to bring her into Negima are:
1. She meets Negi out of class - Too convoluted, considering how much he hesitates to trust people to help him.
2. She knows Negi beforehand - If you're willing to roll with the idea that Seta knows about magic...
3. Make her linked to someone else in the cast: Takane and Anya comes to mind, but my devious one wants to retcon her as a descendant of Evangeline's family. She already has the Mc, just exchange the Dougal for Donell (or don't, really just have Sarah know her family origins). ^^ They even have the same hair and eye color.