Once upon a time AARP represented “retired people.” Because this is a large group, AARP grew large.
AARP has changed its primary focus to “crusader for liberal causes.” You can see this focus in its support of Obamacare and its opposition to Voter ID. Both causes are anathema to a supermajority of its members. As such, AARP will suffer a long, steady decline until its size is brought into alignment with the size of the far-left constituency.
It will take a while for the “sleeping giant” that is America to wake up to the new reality of what AARP stands for, but the change will be irreversible. AARP will never recover from this drastic decline, unless it abandons its extremist political posture.