with what BMD said, I would also switch to a more fishy food (if there is nothing serious with her health first). Here are a couple:
Natural Balance Fish and Sweet Potato
Solid Gold Wolf King
(what I feed)
Wellness Fish and Sweet Potato
Timberwolf Organics' Ocean Blue formula is quite fishy, but it is very hard to find: Timberwolf Organics
This one isn't my favorite, but it's still better than Purina:Eagle Pack Fish and Oatmeal
Personally, unless a doggy has a specific problem or is much less active than s/he once was, I'd stick to the regular food, but feed a bit less. It's just my opinion, of course, and a lot of people do believe in senior foods.
Oh, ya, remember if you switch, always switch very gradually. Purina is much easier to digest (there are things in there to help) than more natural foods, so the doggy's body might need time to adjust to working harder.