AWIN’s database lists 38 737NGs as retired, excluding accidents, write-offs and airframes used for testing, with an average retirement age of 11.7 years. From 2002 through 2007, the same database shows 39 737 Classics were retired, with an average age of 17.6 years.
While the sample sizes are small, there is no disputing that the earliest 737 Classics flew longer than the first NGs built. Eleven of the 39 Classics retired by 2007 flew for at least 20 years. The oldest NG airframe will not reach its 20-year in-service anniversary until December 2017.