Flying hours versus Fatigue Index
Basically yes. 5000 hours straight and level at 20000' is vastly different to 5000 hours bumping around at low level flying down valleys.
Sorties are assigned codes which will signify an approximate level of fatigue for that mission. It is these codes and G meter readings, amongst other things, that will ultimately see a jet consigned to the scrap heap when the time is right.
Different jets obviously have very different airframe lives and this will be related to their construction, role and several other factors.
I have simplified things markedly but you get the idea.
BV