At the ALDC currently on the show, Kalani.
Tessa Wilkinson at CADC is a few months older. (She may be at Jeanette's studio now instead of CADC, depending on how you figure.)
Payton Ackerman is around three years older than either (and a little older than Brooke), but she hasn't been on in awhile.
Oldest ever to actually perform in a competition dance on the show... maybe Kevin Cosculluela, Gino's brother, who "dated" Brooke in that one episode, and put cake icing (?) on her face. Other possibilities are Nina Linhart, or one of the other seniors to dance in that same episode (but not Auriel, since I know at least Nina is older).