I've read the computers in the Voyager spacecraft where built in 1970 and the designs had to be tested with long-term environmental conditions (i.e. very cold) and it took literally years for the testing. So by the time they were launched in 1977, the computers were nearly a decade old.
For the shuttle, it had 5 IBM AP-101s computers - derived from the old IBM 360 mainframes which are radiation hardened. THey were called "System 4/Pi" as 360 is 4 Pi in steradians.
They were used not only in the shuttle, but Skylab, B-52s, and F-15s. 4 had identical software and 3 of them ran the same data redundantly with the 4th one as a backup. As they ran, the 3 would stop more than 500 times a second and compare results and make sure that all 2 had the same results. If there was a discrepency between the 3 computers, i.e. one did not agree with the other 2, the one with the odd results would dump it's results and sync itself with the other 3. The 5th computer ran a set of software that was independantly written from the other 4 computers, and it compares results with the others to make sure there is nothing wrong with the programming of the software.
Also, the shuttle software runs within 1 meg of magnetic core memory.