Processed film does deteriorate, but not at such a rate that surviving 10-15 years would be remarkable! It also depends on the type of film; black & white film lasts longer than colour film for example. (Indeed, when film studios want to store an old colour movie, they separate the colour film into its R, G and B channels, and record each channel separately on its own roll of black & white film. The three channels can easily be merged together again at a later date.)

If you want processed film to last for as long as practically possible (which I would assume people do), then the basic precautions are to protect the film from light, heat and humidity - which can be an easy or a difficult thing to achieve depending on local conditions. I would also advise using archival film storage pages rather than the commonly used shoebox. Clear File and Print File are brands I would recommend.

For some reading on this, take a look at the Kodak publication 'Storage and Care of KODAK Photographic Materials':
http://www.kodak.com/global/en/consumer/products/techInfo/e30/e30.pdf

And for an even more detailed reference, you can download for free the "bible" of photographic archiving/storage, 'The Permanence and Care of Color Photographs: Traditional and Digital Color Prints, Color Negatives, Slides, and Motion Pictures':
http://www.wilhelm-research.com/book_toc.html

PS. Take a look at pages 661-662 in that book (which are pages 674-675 of the PDF) for some figures on estimated number of years for "just noticeable" fading to occur in various Kodak colour materials stored in the dark at different temperatures.