Age and mileage both matter. Not all capacitors are created equal. Wish I had a database of which brands survive longest on the shelf. I have unloaded my share of decades-old electrolytics on fleabay, either because they were types we'll never use, or because they were just too old to risk seeing a failure in a repair job. There are no guarantees, only odds. Mileage reduces the odds of a part being reliable. Heat exposure is the most clear-cut risk. Either caused by internal heating from ripple-current stress, or from being adjacent to hot resistors or tubes. All electrolytics contain critical liquid chemistry, soaked into a layer of brown paper. A rubber plug is what keeps the chemistry inside where it belongs. Rubber lasts longer in the proverbial cool, dark place. Once it shrinks and hardens from heat exposure, the seal is broken and the critical chemistry evaporates. Just saw a Panasonic mobile radio from 45-plus years ago that showed no signs of trouble, but it looked like it spent its life in a carton in a climate-controlled closet. Oddly enough it was full of Panasonic electrolytic caps. They have a good quality record, so that probably improves the odds. Time spent in an attic, basement or garage will age things more rapidly than a dry closet with a constant temperature.
High-quality parts age better than low-budget knockoffs.
A part that checks like new after decades on a shelf probably won't deliver the same service life it would have if it had been put to use 40 years ago. An old part that checks okay today probably won't last another decade or more. But will it be a week, a month or a year? Never have learned how to predict that.
73
