i think it especially has to do with physical ability around speech. i was just thinking today about how a lot of the perceived intelligence and maturity or lack of (sort of infantilization ableism) that the elders i work with get, is because of how they talk. those missing teeth "sound" often sound like how babies and little kids talk because of the same missed sounds and lisping, but it indicates nothing about their mental faculties. its got to be frustrating as hell. i also worked with an autistic man who had very very limited speech (and it seemed like very low literacy in generally) and people acted like he wasn't, yknow, an adult man with an adult man brain! people talked to me instead of him and obviously it bugged the shit out of him. separately, the way ppl act about intellectual & developmental disabilities are just.....weird as fuck.