Acting Your Age
A lot of developmentally disabled people are treated like they aren't really whatever age they are, instead, they're younger. Other disabled people are treated this way sometimes, but it's much more overt when used on developmentally disabled people. Sometimes, especially when the age they supposedly are 'mentally' is extremely young and they're adolescent or older, there is a strong feeling of revulsion associated, as if anyone who acts like a baby without really being a baby is disgusting.
A big part of it, that I see, is the idea that there's only one way to act your age, and that's by acting normal. Even with ages that are often negatively stereotyped, such as adolescence, it is often seen as sad if someone is acting more 'childlike' than like an NT teenager.
But they are their own age. They just don't have the typical way of behaving at that age. Many Down Syndrome kids around 5-8 years or so, for example, have a certain common way of looking and acting, so that it's fairly easy to recognize a DS kid that age as opposed to a different age. Not all DS kids are that way, but most DS kids in that age range that I've met do. But the way 5-8 year old DS kids are is different from 5-8 year old NT kids. They tend to be more playful, more sociable, more adult-oriented, speak less well, are smaller and are less independent. Some of these differences somewhat resemble younger NT kids, maybe around 2-4 years old. But they aren't really like 2-4 year old NT kids, but like 5-8 year old DS kids.
Edit: I uploaded an edit of my changeling lullaby video here: