I started getting their advertisements in the mail right before I turned 50. Their mailings go straight into the trash.
I used to do the same thing until some FREEPER here suggested filling the mailer with some sort of heavy objects and returning it to them so that they have to pay for the return postage.
I've been filling the mailer with old metal washers that I get from a friend who repairs cars. He gives me all of the old washers and I store them until I need to "respond" to AARP mailings. It's very satisfying ...