If nylon strings have deteriorated, you soon know about it - they get crystalline, this makes them opaque, and bits flake off. The bass strings don't suffer as much as the monofilament trebles, so if you find they break up on you it's worth finding a store that has single classical strings for sale and just replacing the trebles. NB: light exposure hastens deterioration, so if you keep the guitar in the dark the strings last longer. The older the strings, the more susceptible they are to light. (Sorry if I used too many long words, Spaw)