My iPhone is about five years old. I am kind of like this guy - planning to get everything out of it before it goes into retirement. The "On" button is not as sensitive as it used to be and I can't install some of the latest applications, but otherwise works just fine. The case for it, however, started to fall apart recently - small plastic pieces are coming off, so I need to get a new one.
My top choice is this "cement" case from Society6. It looks nice on the photo and I hope it does not disappoint in person as it is a bit pricey. I better order it soon before they stop selling cases for older phones.