What if the adult baby did something they should be accounted for but instead but out all their frustration and blame on you? And when you tried to reason with them they refuse clarification?
So this is probably repetitive but I didn't see this question before I answered already. I agree with and reiterate what amanda1983 said on the first page.
I have known people like the "adult baby", and normal adults can sometimes act a little bit like that to some degree. There is no point in trying to reason with them. They are not receptive of it and project their stuff onto others. They probably have narcissistic personality problems. It is not the same as reasoning/teaching a child, because they are not, like the person above said, learning or developing, but they are stuck inside a personality disorder. You cannot win by trying to get them to see the truth or your position because it is not possible. Just make sure you are taking care of yourself, and don't hold on to anger or resentment, because it will only make you sick and unhappy. Don't concern yourself with what the problematic adult thinks.
I once lived with someone who blamed everything and everyone but themselves for their misery, and consciously and unconsciously tried to drag the lives of those around them down into similar destitution. She was a sweet person sometimes, but very toxic, and had incredibly fraught relationships with her children, many of whom suffered terribly, made their own mistakes, and inherited similar mental illness. She created difficulties and dramas and she was the innocent hero of all these stories. It wasn't conscious. It was as if she were a force, and you can't reason with a force. Helping them is sometimes just enabling them to continue their cycle of unhealthiness.
Expecting someone like that to do the right thing, realize their mistakes, do right by you, or give you what you think they should give you will make things worse. Don't bother standing up to them. Stand up for yourself in your own life. If you can't do much yet, work towards your independence, protect your self-worth. I am learning how to do this myself. It's a tough job. Don't give up.