No one seems to be biting so I figure I'll throw in my 2 cents worth...
It's very seldom that you'll ever wear an airplane out. There are airframes out there that have 80,000+ hours on them and have the potential to continue on for many more hours. What typically happens is that they lose their economic viability and it is no longer cost effective to keep them in the air.
As long as there is money to be made by keeping an airplane airworthy and "on line" operators will find a way to overhaul or refurbish them as required. However, as technology comes on line, new designs are developed which do what the older aircraft do for less money. New technology costs big $$$, so older aircraft do have a tendency to hang in there. For example the B727 was just recently retired from mainline aircarrier service - it no longer made economic sense to keep them around. Will they all go to the scrap heap? They'll be around for years to come in "2nd and 3rd" tier operations. (Hey, you still see the occassional DC-3 and DC-6, etc. hauling freight in 3rd world countries.)
Here in a few months, when DRVSM is implemented in the continental US, you're going to see a bunch of older "1st and 2nd" generation bizjets finally grounded. There are a lot of older 20 series Lears, Westwinds, Jet Commanders, Sabreliners, Hawkers, etc., that are barely economically viable now, whose owners will be unwilling to spent the $100,000+ that will be required to get the aircraft certified to fly up above FL290. Flying those old turbojet powered aircraft at altitudes below FL290 will be too costly. The result - to the boneyard they go. It's been a long time coming, many of them are only worth what their engines are worth - it's not even economically viable to overhaul them.