It really depends on the individual. My mother and my in-laws are all right about the same age. My in-laws are able to get around like people 30 years younger than they are. My mother on the other hand has really struggled with any activity that involves much walking lately. She has to walk very slowly or she can't cover much distance. Even then she tires out very easily and we take lots of rests on benches along the way for her. It is about a million times worse if it is hot (and she has become MUCH more sensitive to the heat lately so even when WE were just hot but not uncomfortably so, she felt like it was almost undoable).
I would recommend planning for what you THINK you will be able to do, but don't worry if you have to slow down. My husband and I have been going together since we were in our 30s and even then, when we could have kept up an insane pace all day, we often enjoyed just sitting at a bench with a snack and people watching. There are also a lot of attractions that are longer where you are sitting in a nice air conditioned theater. Those attractions are good ways of giving yourself a break while still being able to cross an attraction off your list. Hall of Presidents, Mickey's Philharmagic, The Tiki Room, Country Bear Jamboree, Monsters Inc Laugh Floor, Tomorrowland Transit Authority, Stitch's Great Escape, Carousel of Progress, and the WDW Railroad are all great options for taking a break from the walking and heat. At Epcot The American Adventure, The Circle of Life, Ellen's Energy Adventure, Impressions of France, Living with the Land, Spaceship Earth, and Turtle Talk with Crush are helpful (Reflections of China and O Canada are also good but you actually have to stand during the presentation). At DHS try Beauty and the Beast (is it still there?), The Frozen Sing a Long, Indiana Jones show, Muppet Vision 3D, Voyage of the Little Mermaid, and One Man's Dream. At AK Finding Nemo, Flights of Wonder, It's Tough to be a Bug, Kilimanjaro Safaris, and Festival of the Lion King work.
It's WDW - so even if you need to slow down and take a lot of breaks you are still in the Happiest Place on Earth and will have a great time!