[roenxi]

In legal terms, children aren't full humans. They literally don't have fully formed brains and there isn't an expectation that they can make decisions that consider the consequences of their actions.

In the sense that a phrase like "growing autonomy" doesn't really mean anything, sure they should get that. Practically, they shouldn't have a lot of autonomy. The concept of childhood education is largely predicated on the idea that children have no idea what is going on and someone else should be inculcating knowledge, values and beliefs in them while making long term decisions on their behalf. And there is a pretty good argument that those values and beliefs ought be aligned with their family.

[card_zero]

Does the brain form fully all of a sudden at age 18, except in Mississippi where it takes until 21?

[bigstrat2003]

No, but the law is not a thing of subtlety and nuance. It is a thing of bright lines. It would be infeasible to have a law that says "children can make adult decisions when their parents think they're ready", so we have to pick a cutoff point which tries to strike a balance between giving too many immature kids power over their lives, and restricting too many mature kids from making decisions with their lives. Some kids will be unfairly held back because they are very mature at 15, some will ruin their lives because they are completely immature at 18. It's imperfect but no perfect solution is available.