Depends on what it look like - a photo would help. Generally 10 years would be a maximum age subject to storage in the dark but most ropes which have had a lot of use will wear out in half this time.
We had some 8 year old Edelrid Extrastatic (probably the best SRT rope ever made) which had seen little use and it survived 4 drop tests when tested by the BCA.
The Rhoydd rope in Simon's photo looks like some rope we left in there about 6 years ago which was about 5 years old then but was in good condition. It proves that you can never trust fixed rigging.