Percy Jackson, Rick Riordan
Rick Riordan's Egypt series, The Kane Chronicles
Harry Potter, JK Rowling
Savvy, Ingrid Law (and also Scumble)
Real Mermaids Don't Wear Toe Rings, Hélène Boudreau
Linda Archer Buckley's Gideon series (I don't remember the exact ages, but thereabouts)
The Magic Thief, Sarah Prineas (around that age)
MT Anderson's Pals in Peril series (they're in 7th grade, which is when you turn 13)
Plain Kate, Erin Bow (I can't remember the exact age, but it's around there, I think)
Icefall by Matthew J. Kirby MIGHT have a protagonist that age
Geeks, Girls, and Secret Identities, Mike Jung
Splendors and Glooms, Laura Amy Schlitz
A lot of fantasy novels involve kids coming into their powers when they turn 13. Some of these might hit a little low or high, but they are all *around* that age. Um...I'm sure there are more that are obvious, but I'm drawing a blank at the moment...