The Boeing company does not seek to defend it's aircraft when researching accidents. It is very interested, however, in seeking to make it's aircraft as safe as possible. After the two consecutive crashes of 737 aircraft, at Colorado Springs, and Pittsburgh, respectively, the Boeing company spent millions of dollars, over three years and over 75,000 engineering man hours in an intensive investigation.
I have had the opportunity to talk with many industry veterans, two of which former 747-100/200 pilots, and all have explained that in their opinions, the missle theory is most probable. I do know however that the explosive fuel tank problem does exist in several types of aircraft other than the 747-200, but do not believe this is what brought down TWA800.
Also, I believe the age of the aircraft was not an important factor here.