I don't know how they decided to set the age at 50, but personally I am glad they did. Retirement is not that far off. AARP has many members and is becoming an increasingly powerful lobbying group (which will only become more so as the baby boomers age). So why not get in on the ground floor being informed about what is ahead and knowing what issues are hot so you can add your voice.
Besides all that . . . look how many places offer AARP discounts!!!