You're asking about two very different groups---tourists and seniors. Some casinos cater to out-of-towners; those would be the Strip casinos. Others cater to locals, which includes a fair number of seniors. I can't really come up with a casino that specifically targets senior visitors.
Most of the locals' casinos are supported by a large group of seniors and as a result, cater to that demographic with promotions and offers. If those casinos are smack-dab in the midst of suburbia, that's where you'll find the senior action. However, unless you have a car, there's no way to get there.
Of those locals' casinos, Emerald Island in Henderson, the two Cannery casinos, and the two Arizona Charlie's seem to me to be the most senior-friendly. The two big locals casino chains, Boyd and Stations, used to have great promos for seniors on weekdays. Now, they've both cut back drastically on promos and tightened up their games. Oh, one more possibility is Rampart in Summerlin, which gets virtually zilch tourist traffic due to its location and is surrounded by well-off retirees. Their promos continue to be pretty good.
If you're visiting and don't have a car, you'll find downtown to be much more senior-friendly than the Strip, with lower room rates, better gambling, and good food at bargain prices. Stay the hell away from Circa, but otherwise, it's all good. Check out the California and the El Cortez.