In all honesty, i think even letting elders use a walking stick (not just elders but anyone teen and up) with a unique walk style would be a good start. Even a walking frame would be cool. Disabilities is something I'd definitely write into my stories (I'm a story based player), although it isn't high up on my list of features I want in the sims 4.
I think through the most recent videos on players who use the sims 4, that adding disabilities to the game could potentially actually help people 1. explore what it could potentially be like for people who have them
2. gain empathy towards people who are disadvantaged and the struggle they go though
3. for people with that particular disability to be able to normalise their sims to their own reality
I dont see disabilities as a bad thing to add to this game, so I'll support the idea & purchase it if it were to come out.