Most of the problems with audio tapes come about through repeatedly playing them. If they're stored in dry conditions, and not played very often, they last for ages. (I've got some reel-to-reel tapes recorded half a century ago, which still play as well as they did when they were first recorded).
However damp storage conditions can lead to mould formation, leading to the tapes becoming unplayable. Also, with some tapes, the lubricant layer can dry out over time, leading to the tape becoming brittle.
For a far more detailed answer, see here:
http://www.clir.org/p...ub54/2what_wrong.html
Chris