Wirelessly posted (BlackBerry: BlackBerry8530/5.0.0.1030 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/417)
The AARP discount is a discount off of a regular, published fare. The AARP discount does not convert a regular fare to a senior-citizen fare. (I'm not even sure that BA even offers senior-citizen fares currently.)
If you have -- or can get -- the Chase/BA Visa card, that card provides a 10% discount on TATL BA fares. Although the offer terms state that the discount cannot be combined with any other "offer," some FTers have reported being able to stack the Chase/BA discount on top of the AARP discount. Details on the Chase/BA discount here:
http://www.britishairways.com/en-us/...visa-signature
YMMV.