King Daniel Into Darkness wrote:
The Klingon D7/K'tinga design has been used for at least 225 years (first appearance: ENT: "Unexpected" in 2151, seen throughout TOS, TNG and DS9) and the Bird of Prey for almost as long. Although I don't know how long each individual ship is supposed to last. Their rusty, worn out appearance suggests they're very old.
Indeed, however "design" doesn't mean "ship".
On other notes. As was said before the USS Enterprise(CVN65) was in service for 50 years. Had there been more Enterprise-class ships I don't think she would have been retired.
Also, the USAF B-52 Stratofortress line is STILL in service after more then 60 years. The plan is to keep them going to at least 2045. Thats 90+ years of active service.