Hey y'all

I honestly can't figure where or how I heard that PSUs will degrade in power output over time. It was a quite a long time ago aparently. So I'm trying to find some proof or reasoning behind this as it has come up in a few discussions/threads over in HardForum in the past few weeks. My google-fu is a bit weak at the moment so I'm hoping someone can provide me key search words or even full on links showing how or why PSUs will degrade in power output over time.

So does anyone have any links or articles showing how or why PSUs will have a degraded power output over time? The closest I found so far was this thread made a few weeks back but wasn't entirely what I was looking for:
http://www.jonnyguru.com/forums/showthread.php?p=68491

Thank you.