Or, just google "BA AARP Discount" and the first unsponsored link takes you here:
https://www.britishairways.com/en-us...ember-benefits
Read the T&Cs. Hint. #12.
ETA - that said, the T&C's don't indicate anything about the discount being valid for First Class, and we know that it has been, so YMMV. I suppose someone with an active membership can double-check. Mine lapsed recently and I may not renew, pending decision on who/how to fly to Europe on my next trip.