BA are not exactly known for a 'young fleet' policy unless they are replacing something completely uncompetitive. The 74 Clasics used on the JFK run in their last years were literally falling to pieces in their last days and must have ben about 27 years old. As Mr E has said the 744s are around for along time to come.
I would have throught that having a decent sized triple 7 fleet there was a lot to be said for a mix of Standard 777s (RR engined) LR ones and 300 s to replace the older 744s.
Also gives BA a nice middle of the road position with suppliers SH=Airbus 320 series which vastly superior from Pax view than Boeings. LH = Boeings mostly 777, which while not my fave plane to look at (hardly matters does it) seems to do the job pretty well.
Looks to me like BA have got a good fleet strategy -finally- and have the option of adding 'debugged' 787s or 380s maybe around 2010 without any short term risk