That was my very first thought when I heard about this - the shuttles are old machines. Now I know that commercial aircraft, with proper maintenance, can last 30+ years before having to be taken out of service. However, it seems to me that the stresses put on the shuttle's airframe would be much greater than those placed on a Boeing jet, causing the frame to wear out more quickly.
As has been said elsewhere though, everything at this point is idle speculation.
I can still vivdly remember where I was and what I was doing when I heard about the Challenger explosion - that was something I surely didn't need to have brought back to me with this. May God bless us and the families of the Columbia crew.
On the other hand, they fly less often, so it may even out.