Non-US properties often don't have an AARP rate. A number of properties (foreign and US) have a senior rate. Some properties don't have either one. The age for most senior rates I've seen has been 65 for some time.
I read that you can use Hilton Points on Amazon?
Years ago Hilton had a Senior Honors program which had a membership fee and used 60 years of age as the eligibility criteria.
Senior Honors®: The Definitive Thread