Interesting article. I'm sure all airlines, and in fact all operators of hi-tech machinery, face the same quandary. Technology is advancing so quickly. Of course, the fact that fuel has trebled in cost in five years is a major incentive.
When the article say that American's fleet of MD80s averages 18 years old, I think they must be including later derivatives as well, such as the MD90. Otherwise I don't see the 18 year average as likely. The MD80 itself was last made almost 18 years ago.