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.