My daughter received an AARP card 2 weeks before her 20th b'day. My husband began receiving cards from the age of about 48, but I never gave them to him because I didn't want to make him feel old- one day he did something to annoy me so I decided it was time to give him ALL his mail. He got back at me though - he enrolled ME!!!! We have never received more junk mail than we do from this organization and the discounts aren't anything to write home about. If it's discounts you want try AAA.
Personally I think AARP just buys address lists and does mass mailings to fish for new members.