Why is a hypothetical child very often referred to by the pronoun "it(s)" in in contexts related to the child's lack of autonomy? Can you think of any contexts where an adult or adolescent would be referred to that way?

When it comes to discussions and publications regarding children as a human age group, I very often - in singular contexts - see "its" used to emphasize on their lack of autonomy. Here's one example: "The child harness serves to keep the child from escaping from its parents".

I don't get it. Children are humans with the ability to think, walk, talk, and a lot of basic survival instincts. I can understand "it" being used in reference to an infant or toddler since they barely have an autonomy, but by the time they'd be past both those stages and just referred to as a "child" (in the context that relates to age group, not being their parents' offspring), they have more autonomy than they did at 5 months, even if not to the same extent of a ten or twelve year old.

Again, is there any conceivable context where "its" can extend to an adolescent or adult?​