It varies from hotel brand to hotel brand apparently.

The Homewood Suites in Anaheim doesn't have the traditinal AARP box, but it has a Senior Rate* box, with no * footnote on that page. But if you click through to get the actual rates page, there it does explain that the senior includes AARP as a qualification. (In fact, on that page, it shows as "SENIOR/AARP".) Tho in this case it's identical to the AAA rate...

I did a search around Palm Springs, and once again Homewood Suites in La Quinta had the same box. (Other than Homewood and Hampton, I didn't see any sort of AARP option.)