Okay but for navies of developed nations, we still have a ways to go to beat the USN
USS Lexington (CV 16)
Commisioned Feb '43
Decommissioned Nov '91
48+years (mind you she was out of service for a period following WWII)
I actually stumbled on this fact when preparing MRLs for the Iroquois class when I was in Ottawa.
Then there was the USS Michigan. She operated solely on the Great Lakes and was renamed the USS Wolverine in 1905. She was commissioned 29 Sep 1844 and decommissioned 6 May 1912. I came across here while researching the 1866 Fenian incursion into the Niagara Peninsula, as she intercepted and apprehended the Fenians as they attempted to escape to Buffalo.
USS Constellation (1854) http://en.wikipedia.org/wiki/USS_Constellation_%281854%29
USS Constellation, constructed in 1854, is a sloop-of-war and the second United States Navy ship to carry the name. Decommissioned in 1933, Constellation was recommissioned as a national symbol in 1940. Constellation was again decommissioned on 4 February 1955, and stricken from the Naval Vessel Register on 15 August 1955 – about 100 years and two weeks from her first commissioning.