I am sure there is some older 737 fleets around the world. Infact if you look at this web page
Boeing 737 Production list - Airfleets
It list all the 737-100/200/300/400 and 500's built and there current status
It shows the oldest 737 still in service is a 737-222 which is currently with
Air Philippines . It flew it's first flight on 30th November 1968. Meaning it will be 40 in six weeks.
The oldest BA mainline 737 in service is a 737-300 registered G-LGTI. Which flew it's first flight in May 1988. This aircraft will leave the fleet in February 09.
The youngest is a 737-500 reg G-GFFI which flew it's first flight in September 1995 and will leave BA in March 10
Found the info on when the aircraft will leave BA fleet is from :
British Airways Boeing B737srs
Hope the info above and the info on the sites above will answer your question