How old do you have to be to join AARP?

You don't have to be any particular age. They ask for your birthday, but require no proof. If you enter a date that makes you at least 50, they let you join.

The above is only partially correct. Further is-- you don't have to lie or omit the truth. People of any age can get an "associate membership," which is devoid of some, but not all benefits. ( left unspecified on the AARP site)