M_Boats: WHat is the primary reason sails wear out?

Is the length of time a sail is useable based on age? TIme under sail? Amount of wind sailed in? All the above? Or mostly hours under sail...whatever conditions might come? My sails were "parked" for 27 years in the cabin in the sailbag. How do I know if my Monty sails are giving me the best performance? Kendall M-15 #164