Average Cruise ship life is usually 20 to 30 years in operation. New safety rules and increasing competition prevent longer life spam of cruise ship and force them to early retirement. However there are exceptions to this rule like the oldest and most luxurious cruise ship, Sea Cloud sailing the seven seas for more than 80 years. Without regular maintenance and proper care these vessel would not have survived so long.
Oldest cruise ships still sailing are 50 to 80 years old, and two out of five are property of a single owner specializing in classic cruise ships. Classic International Cruises charters this ships to cruise operators that like to offer their costumers something different from today’s floating resort ships. 
This is the list of five oldest cruise ships still sailing

Sea Cloud (1931)

Sea Cloud was commissioned by the Wall Street broker Edward Hutton. The ships original name was "Hussar II" and was build in Kiel, Germany. At that time this was the largest private yacht in the world
In late 70s the yacht was in a very poor state and after extensive restoration in 1979 Sea Cloud was born Since then, the Sea Cloud is owned by Hansa Treuhand from Hamburg and is operated by Sea Cloud Cruises.
•    Owner: Hansa Ltd. cloud.
•    Overall length: 109.7 m
•    Width: 15 m
•    Passenger capacity: 64
•    Commissioned: 1931
•    GT: 2,523

Athena (1946)

This ship with not very flattering appearance entered the history books when it collide in a heavy fog with Italian ocean liner Andrea Doria. Fifty-one passengers and crew were killed in the collision. Athena was build in Gothenburg, Sweeden under her first name Stockholm. The ship was rebuilt, renovated and sold countless times.
in 1960 she went to the Free German Trade Union Federation FDGB the GDR, and served them for several years under the name of Peoples' Friendship as a cruise ship. From 1966 to 1985 Stena Line was operating the ship during the winter season in  Caribbean islands.
before it was sold to Italy in 1993 the ship was used for asylum seekers accommodation, in Oslo in 1986 under the name Friedjof Nansen.
Finally, after extensive renovations in 2005 the ship was sold to Classic International Cruises and got its present name Athena.
•    Owner: Classic International Cruises
•    Length: 160 m
•    Width: 21 m
•    Passenger capacity: 552
•    Commissioned: 1946
•    GT: 16 144

Princess Daphne (1955)

Originally this was a cargo ship named Port Sydney traveling British ports. In 1975, she was completely rebuilt and turned into a classic cruise ship named Daphne.
After several ownership changes, the Monegasque company Dream Cruise Line Cruises, the ship was renovated in 1996. Meanwhile, the ship ran under the name of Switzerland, Ocean Odyssey and Ocean Monarch. Since 2008, the ship is traveling again as Princess Daphne.

•    Owner: Classic International Cruises
•    Length: 162 m
•    Width: 21 m
•    Passenger capacity: 550
•    Commissioned: 1955
•    GT: 15 833

Funchal, Madeira (1961)

The Funchal was originally commissioned for Portuguese society insula de Empressa Navagacao as a ship for delivering mail and transporting people on the route Lisbon - Azores - Madeira. For a vessel of this age is rather unusually keep its name without interruption to this day.
The ship was rebuilt in 1973, replacing steam turbines replaced with modern diesel engines.
•    Owner: Classic International Cruises
•    Length: 154 m
•    Width: 19 m
•    Passenger capacity: 471
•    Commissioned: 1961
•    GT: 9563

Hebridean Princess (1964)

This cruise ship was originally build as ferry boat under the name of Columba.
Columba built in 1964  served its first years as a ferry to the Isle of Mull, and later other Scottish islands. In 1968 she was converted to take 600 to 870 passengers.
After extensive renovation in 1989, the former ferry was introduced as luxury cruise ship Hebridean Princess and began trotting around the islands of Scotland.
Today Hebridean Princess has very specific itinerary , mostly near the coast of Scotland, Ireland and England.
Hosted a special passenger ship for nine days in July 2006: Queen Elizabeth II chartered Hebridean Princess for a royal family vacation to celebrate her 80th Birthday. The ship must have pleased the Queen, because in July 2010 she came again for two more weeks.
•    Owner: All Leisure Group
•    Length: 72 m
•    Width: 14 m
•    Passenger capacity: 49
•    Commissioned: 1964
•    GT: 2,112