As others have said, Yellowstone is very tourist friendly. There will be a few places if you want to see certain waterfalls and other vistas that will require going up and down stairs (and they may be slippery).
The rangers are plentiful, visible, accessible, and approachable. If there is one present and you have any questions, follow their advice. Stay on the path and don't take selfies with the bison
Aslo from an ecology and conservation standpoint, very little of Yellowstone is developed and it's to protect the land as-is so it's in everyone's interest to obey all signage and ask ahead if you are unsure if a path is suited for your abilities. But the facilities that exist are great!