Last time I checked (admittedly a few years ago), AARP was in bed with the pharmaceutical industry. After all, plenty of seniors are loaded with drugs to handle all sorts of conditions. There were financial incentives to AARP to promote pharmaceutical drugs (particularly in AARP publications), IIRC.
So while there are benefits to joining available for anyone, AARP is big (has many aspects) and may or may not always serve your interests.
If anyone already a member can invistigate this further and/or correct me, I'd certainly appreciate it.