Determination of aged aircraft
 

Will somebody kindly explain to me which parameters are checked when retiring an aircraft.I notice that even the A320 now has retirees lately.I have a faint understanding of metals but i can never get the definitive reasoning for retiring an aircraft.Engineers design an aircraft for a number of flight cycles but I do know that this magic number is often exceeded during service life albeit closely monitored by engineering departments.Were it not to be monitored,would parts fail Hollywood style?
I am not referring to older aircraft with thirsty engines where economics is the main factor.