From Cornell Dubelier (who is one of the oldest capacitor manufacturers):

“Aluminum electrolytic capacitors stored for more than 5 to 10 years may have increased levels of DC leakage current. Check if DCL meets application requirements before placing in service. Recondition high DCL units by applying rated voltage through 1,000 Ω resistor for 30 minutes. Shelf life is a measure of how the capacitors will withstand storage for long times especially at high temperature. To test shelf life place the capacitors in an oven set to the shelf-life test tem- perature –0 +3 oC for the shelf-life test period. Upon completion of the test stabilize the capacitors at 25 oC for 24 h or more. Apply the rated voltage for 30 minutes, then verify the post test limits.”

And from Nichicon:

“When an aluminum electrolytic capacitor is stored under no load conditions for a long period of time, its leakage current tends to increase slightly. This is due to a drop in the withstand voltage of the dielectric caused by the reaction of the anode oxide layer with the electrolyte. When the voltage is applied to the capacitor, the leakage current returns to its initial level because of the re-forming action of the electrolyte (called voltage treatment). If the storage temperature is high, the leakage current will increase substantially. Therefore, it is desirable to store capacitors at normal temperature level with no direct sunlight. A voltage treatment is recommended when using a capacitor stored for a long period of time. The treatment for an individual capacitor is accomplished by charging up to its rated voltage through a resistance of about 1 kΩ and applying the voltage for approximately 30 minutes.”

The consensus from manufacturers seems to be that there is a small, but measurable degradation in performance from long shelf life, but unless the electrolyte has dried out completely, the capacitor may be “re formed” by applying the rated voltage through a current limiting resistor for approximately 30 minutes. LONG answer to a short question 😂

Depends on the type of capacitor. for Electrolytic capacitors (with a positive and negative polarity), it is good practice to charge them periodically. You may find this article useful: http://www.qsl.net/g3oou/reform.html
Oil filled capacitors (non polarized) typically are fine even after long periods of non-use. I have a large 8KV 16MFD oil filled that was originally a photoflash capacitor in my ham radio high voltage (4KV) power supply, and it was probably unused for 10-20 years when I first charged it, and it works fine.