You could probably have that conversation a hundred times with a hundred different people and get a hundred different ideas, responses and conclusions.
( My opinion)
The answers are very simple you can take a child within the first years of development and then take an elderly person within their final years of existence and you’ll see surprising similarities.

They’re both experiencing limited control and understanding of their bodies and environments.
As well as limited communication skills.

The child is feeling physical pain from growth and development. They can hear and see but lack the knowledge to communicate with formed words and then add in any sense of abandonment, fear, or frustration from soiling oneself and a child will mostly display some form of negative behavior which since we can’t remember what it was like for us at that stage of development we are usually unable to fully connect and/or grasp what is exactly the source of their behaviors.

And in the elderly adult who has lost significant levels of their sight and hearing which will result in a dramatically lessened ability to communicate as well if they’ve also suffered any form of mental deterioration. Also factor in the similar issues that they would share with the infant such physical pains, feelings of confusion, abandonment and the fear of not knowing themselves or their environment as well as soiling oneself.

But ironically no one diagnosis them as being autistic.
I guess what I’m saying to everyone one is imagine if it were you under those circumstances and knowing that in most cases it’s simply that we don’t when we attempt to determine what’s wrong with a child.
And all we really need is a solution for us and a label for them so that the rest of society won’t blame us for not being able to the problems of an infant or the elderly.