Perhaps it depends on the property or the timing of local events. We have made some family trips, getting a few rooms and my mother-in-law has been able to get a senior discount. I know Marriott doesn't give AARP discounts but usually the AAA discount is pretty good and one I leverage often.
Being a senior, I too have noticed that there are some hotels that do not offer a better rate for seniors. I also have noticed that some of the hotels do not show up in the search at all if you select the senior rate option in your search. When I do my searches, I always try the search multiple ways i.e. with bonus rewards, with special rate, and without either. This is the only way that I have found to search to be sure that I am getting the best rate possible. Another trick I have to get the most out of my dollars is to do the search and check the box next to "My dates are flexible". You can also select a "Special Rates and Rewards" but this is where I try the search twice, once with the "None" as the option and once with "Senior Discount" as the option. Once you have the search form fill out the way you like, click "Find". I will do my search on an area to get a list of all the hotels in the area. Once you have the list, you can sort by "City" or "Brand" using the sort by button in top right of page. I usually sort by brand because I stay primarily at the Courtyard, Residence Inn, or Fairfield Inn. Then click "View Rates" next to a hotel that interests you and you will be taken to a calendar. You can then scroll through the months and get the lowest rates based on time of year. In my case, I want to plan a trip to Sanibel. If I go in March, the room is $271.15 to $305.15 a night, however, if I go August 14th to the 24th, the rate is $109.00. Big difference! I posted some screen shots below that will hopefully be of help to you. I always remember that I am probably not going to get the best room selection when I go this route, but many times I get an upgrade...it all depends on how booked the hotel is.
My personal experience with the senior discount is that in most cases it not only is less expensive than the lowest rate listed on marriott.com (including the Advance Purchase Rate), but that it also can be cancelled until 24 hours prior to the reserved stay. At some hotels the senior rate is the same as the Advance Purchase Rate, but instead of being "non-refundable if cancelled more than 1 day after booking", the senior rate can be cancelled until 24 hours prior to the reserved stay. There have been very few instances where the senior rate that was available was the same as the Best Available rate (which is refundable). The senior rate is usually available for only a limited number of room types.