There's nothing wrong with older aircraft if they are well cared for. And sometimes the build date is quite misleading. An aircraft with hushkits or a different engine, avionics updates, and thorough refurbishment and mechanical inspections/repairs can fly reliably for a long time, and in fact be quite modern. The big factor to keep in mind is not the fatigue life of the airframe, however, but rather the economic life. At some point it does become worthwhile to buy newer equipment. But in actual fact, when it comes to structure aircraft haven't changed a whole lot for 30 years, so other than some refinements of the newer models there isn't a huge difference to really make old ones obscelete.