electrolytic cap shelf-life after being mounted on circuit borad
Shouldn't make any difference whether they are mounted or not. 

The main factor in electrolytic capacitor lifetime is what temperature they are stored or used at and for how long at high temperature. Rule of thumb is that life halves for every 10°C rise. 

So if they were stored in a cool place they'll have a long shelf life. If they were stored in an attic in Phoenix during the summer, less so. 

Voltage was applied during testing at the factory (and before that to the raw etched foil), and it certainly leaked off within a short period after that.