MeeMee raises a good point. I firmly belief that a lot of the neurosis and pain that we go through in our adult years comes from the false information (or missing information) from our childhood. People are obviously trying to "protect" kids so they hide the fact of death, they don't let them attend funerals of loved ones, they make up euphemisms to describe where a dead loved one went, and on and on and on.
Then you get to adulthood and everything feels like an unfair shock to you. Not just with this issue but a whole host of other issues too. Death is treated like an aberration. It's not! It is sad, it is painful, we miss our loved ones and especially when they die at a time we don't like (i.e. "they died too soon") but the truth is nobody gets out of here alive. It's the way the system works! So the sooner the kids deal with the reality of that, the better. Yes, they will be sad, but they will survive, and learning to process huge feelings of sadness (rather than having them swept under the rug) is a valuable thing that will strengthen your child, actually. I think that not having information, or suspecting that you have been given half-truths or sugar-coated info, causes great anxiety. (BTW I am talking in generalities here, not directly to YOU about your child....I'm sort of complaining about how I was raised to not-deal with stuff, which just caused more problems later.)
I stick with my assertion: keep it short and sweet. Your kid wants to know why, you tell her why. In doing so, you're indirectly assuring her of something she is NOT verbally asking, which is, "Momma, are you going to be honest with me? Momma, can I trust you to handle this? Is everything going to be OK?"
I liked my answer "cats' bodies aren't made to last much more than 18 years. (it's true; the range is, I think 16 or so for domestic cats and 20 for the siamese and similar breeds) Because it's true. Cats have a certain general lifespan. So do humans. You don't need to go into how sometimes we get mowed down before that lifespan is up. Just keep it short & sweet. The more we talk, the more we sound like we're convincing ourselves, and that can't inspire confidence in the little ones.
So on that note, I should stop talking already. :-)