How do I keep track of how old food is?
Label everything with the date it goes in the fridge. That's about all there is to it. In my house we keep a marker and a roll of painter's tape in a drawer near the fridge, which makes labeling food easy; then you just stick to the system, religiously.

Incidentally, if it's not instantly obvious what's in a container (e.g., you have a container with opaque sides, or similar-looking substances like gravy and soup), put that information on the label as well.