I've been with Parents in law who were in their 60s and they had a great time.
Like people of any age ,some people of that age will readily get involved in thrill rides, waterpark slides and other ultra active stuff, others will prefer the quieter attractions. The %ages of thrill seekers may be a little different but WDW is a playground for all ages and I'm certain they are going to find a lot of things they will enjoy. You ( or they) may have to adjust the pace of the group a little, do a little extra planning and build in a few extra breaks/downtime. It's much better for them to suggest you move on to a vigerous activity than it is for them to feel they need a rest but don't want to ask for one in case they slow you down or are being a hindrance.
You may find they would appreciate some time on their own in Epcot to explore some of the more educational/informative attractions ( presuming you have kids who may have a limited attention span) , other things they may like would be Cirque du Soleil or some/one of the "behind the ears tours" , my out-laws really enjoyed the tour of the land pavilion in Epcot that explained much deeper all the hydroponics,fish farming and research that was going on in that area. There are a number of tours that go on in all the different parks and depending on their interests one or more of these might be something they would REALLY enjoy.
I agree with Helen and Maureen, allow them to set the pace, make sure they get PLENTY of chance to keep hydrated and encourage them to explore some of the parts of WDW that maybe aren't a priority to the rest of your family yet but are going to be particularly suitable/of interest to them.