There is no age limit for joining AARP, just pay the fee and you can access all their shopping discounts. One is a decent discount off of BA flights, although it's not as generous as it used to be. I think it's $200 off of business class now (used to be $400) and $100 off coach (used to be $200), and it's money right off the top, so if you hit a BA sale, you can get IAD-LHR in business class for around $1600, sometimes less. Not chump change by any stretch but considering coach fares have been running $700-900, it is definitely worth the price difference. BA's business class is not bad, slightly better than the USA-based carriers but not in the same league as Virgin (although it's rare to find business class fares on Virgin for under $2k on that route) or the Asian or Middle Eastern carriers.
With my husband being British, we've been using that AARP discount for years to avoid coach when going to visit his family. I was kinda pissed when the travel bloggers started promoting it because people started abusing it, and then they cut the discount back.