Ivan, the shuttle's airframe was originally certified for 100 flights; however, the
shuttles were intended to fly much more often. The Columbia was therefore like
an automobile that had only been driven 20K miles but was 30 years old.
There's a reason why car makers say five years or 50,000 miles,
There was also a refurbishment/refit in 1991, as I have read on this thread. I would love to have such a car.