The dinosaurs in JP have a hodgepodge of filler DNA that lets them do stuff like change genders. A cloned Rex having a longer than average lifespan compared to its natural born cousins millions of years ago is not out of the question. Even if that isn't true, an average lifespan is just that - an average. With no competition from other predators on the island she could easily live past 30.
EDIT: I should also mention that worldwide only about 50 Rex fossils have been found, most incomplete. I don't think that's enough to come up with a reliable average.
We both know that ended with JP 3, which still sucks.
Is that why the most iconic image of the franchise is the Rex roaring in front of a falling Jurassic Park banner? Or why the logo prominently features a Rex skull and
only a Rex skull? Or why up until JP 3 the Rexs always had more screentime than the raptors?
There's no reason to think she wouldn't. There are no other predators on the island that could have competed with her. Hell, at Site B the Rexs were happily reproducing and the ecosystem seemed to be thriving.