SayUncle notes that someone is going to try an experiment, in regards to whether keeping magazine spring loaded causes them to wear faster. I should note that from engineering principles, there is no doubt that a loaded magazine will wear faster than an empty one, but the question of whether we’re speaking short time scales, such as 13 years, is another matter. I’d be willing to be your time scales here are on the scale of decades.
My feeling, just based on some things I know about springs, is that magazines which get used regularly are going to wear the fastest. Cyclic compression is the most fatiguing to the metal in a spring. Extended periods, even at full, static compression are going to be less fatiguing than if you were unloading the magazine every day.
If I had to bet, the magazine that’s been loaded for 13 years works just fine. At least not much worse than a magazine that’s been empty for 13 years. Time wears on springs too, whether you compress them or not. My feeling is that it’s probably best to leave magazines seldom used unloaded, but it’s far better to leave a magazine loaded, rather than loading it and unloading it regularly.