Film photography is imaging via a chemical reaction to light. The chief ingredient that does this trick is salts of silver. These are chemical compounds that darken when exposed to light. In the camera, we briefly expose the film to an image of the outside world, cast by the lens. This speedy exposure is insufficient to darken the film however we can treat the film with a chemical bath called a developer. This fluid takes over and finishes the job of creating a photographic image.
Photo film is a perishable. Over time, the silver salts change and become developable. Film ageing is a gradual process. Given time, all of the silver salts on the film will blacken when treated with developer. We say the film was “fogged”. Just how “fogged”, is function of time and storage conditions. This action is accelerated by heat, humidity, background radiation, and the like. You can purchase outdated film but the degree of harm is unpredictable until developed.
Now for the advice: The negative/positive film process is a two-step technique. We take the picture using a negative film. All the objects on this film are inverted as to their degree of blackening. Further, this negative image is virtually useless unless printed (forming a positive image suitable for viewing).
An opportunity for artistic alteration: You can easily manipulate this process and make images that simulate the aged look to desire. This will prove to be more satisfactory because you can control the results. All you need will be the skills needed to adjust the final image, I strongly suggest that this method is the best path for you. In other words, fresh materials and extermination to get the look and feel that is your goal.