The only two shuttles left in active service is the Atlantis, and the Endeavour. The Endeavour made her first flight May 7, 1992. The Atlantis, on the other hand, made her first flight October 3, 1985, So she is significantly older then the Endeavour. The Enterprise was retired and is now at the Hazy center, just outside Washington D.C., and the Columbia and Challenger were lost. So out of six, two remain in service, the youngest two of the fleet. Below is a breakdown of the shuttle fleet:
Enterprise; Built in 1977, she never actually flew in space.
Columbia; First flight was April 12, 1981. She spent over 300 days in space. She was lost February 1, 2003, when she broke up on re-entry.
Challenger; First flight was April 4, 1983. She spent just over 62 days in space. She was lost January 28, 1986, at just three years old, during the launch process.
Discovery; First flight was August 30, 1984. She spent over a year in space all together. She retired today after 27 years of service.
Atlantis; First flight was October 3, 1985. She has spent over 293 days in space thus far. She is slated at this time to make the final shuttle flight this summer.
Endeavour; First flight was May 7, 1992. She has spent over 280 days in space thus far. The youngest ship in the fleet, she is slated to make the second to last shuttle flight this spring.