Someone forcing their way across your boundaries, what childhood abuse is all about, can lessen a child's sense of being able to enforce their boundaries at all. So, not only does a child not know or learn how boundaries work (because an abuser isn't going to be able to teach them that), they may feel the right to boundaries or any sense of/point of strength in one's self to defend them, even if they knew how/wanted to. It's like having a town under siege get its walls knocked down; the battle's done, the citizens are conquered and give up hope.

But, what one can/cannot learn or do as a child is different from an adult perspective. The adult can learn how to enforce their own desired personal boundaries and can learn how boundaries work and why they are essential for getting along well with others.