How do i know when i can reroof, shingle my roof?

When is it a good idea to reroof my house? My roof does not leak and the shingles are not curling. They seem to be fine. However, they have faded big time and there are runoff stains from the exhaust. it looks horrible.So i really just want to reroof for the looks. Plus, since my house is only 700 sq ft it shouldn't cost me much. I was just planning on shingling over the old since they look fine. No soft spots either. so is it a good idea or just wait until there is a leak? i don't know how old the shingles are. House is 60 yrs old.