(FORTUNE Magazine) - He's Jim Aley, he's 40, and, yes, he just joined AARP. Turns out that this organization of more than 36 million members is ready and willing to take in people who aren't "over 50," though that description still fills its literature and Web site. All you need to join is $12.50.

FORTUNE caught on to these surprising facts when a direct-mail pitch from AARP came to Aley, a FORTUNE editor. The first sentence of the letter said, "Our records show that you haven't yet registered for the benefits of AARP membership, even though you are fully eligible."

Neither shy nor retiring, FORTUNE editor Aley, with his children David and Lucy, is an associate AARP member.

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So Aley sent in the application, scrupulously giving his birthdate as Dec. 25, 1965, and waited. Back came word that he was now an "associate member" - what AARP labels under-50s on its membership rolls.

Wanting to test the limits of this policy, another FORTUNE editor, Peter Petre, went to AARP's Web site and successfully signed up his 22-year-old daughter. We then retrogressed into the grammar-school set.

"How about my 7-year-old grandson?" asked this writer, as she talked recently to AARP executives.

Answered chief operating officer Tom Nelson, "I don't know how good his handwriting is. But if he sent in an application and put his birth date down accurately, we'd say to him, 'Grandson, you can be an associate member, but that's it.' "

So what would an associate membership do for my grandson? He'd get the organization's publications, including AARP the Magazine, but nothing else: no chance to buy auto insurance at AARP's rates, for example, and no opportunity to have AARP "standing up for his rights" in Washington, as it exercises its vaunted political power.

Don't think that AARP welcomes associate members because it makes money on the $12.50 annual fee. It's only when full members buy things like auto and health insurance that AARP begins taking in serious revenues, like commissions. AARP says people under 50 account for less than 1 percent of its membership, adding that Aley's invitation to join probably came from a flawed direct-mail list.

Why not politely turn underage applicants away? There are legitimate reasons, says Nelson, 56, why a young person might want to join. He himself signed up when he was younger than 50 because he was working in the gerontology field.

Another reason for accepting those under 50 could be that AARP needs the members, immature or not. Its membership has grown since 2000 by only about 7 percent, while the number of 50-and-overs in the U.S. grew in the period by 14 percent.

Maybe that disparity helps explain why membership-development director Lin MacMaster says, "We're thrilled that your colleague joined."

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