Car rental companies are allowed to refuse (or charge more) for under-25 drivers because of insurance risk where the practice is not forbidden by statute (i.e. NY). They have data that can support the age requirement is reasonable, and as such, the likelihood of success for a discrimination lawsuit would be remote.
I would think they would have a harder time coming up with supporting data for the 75+ or 80+ (or higher) age group, and that group is smaller compared to the rest of the pool of available renters. Since their insurance likely is not impacted materially by including this group, they choose not to take any action.
-ryan