Your frontal cortex isn't developed "fully" until your 25, Does that mean until you're 25 you are unable to make rational decisions? No. The age of consent is just an idea we fabricated in the last century when we decided to tie it to education. We somehow decided 18 is some magical number that in reality has no meaning when it comes to maturity. Children over 6 years old are perfectly capable of making their own decisions regardless if you think otherwise. The government denied rights to children for countless of lifetimes for the same reason, And that reason is no longer acceptable. Due to our modern age of technology information is accessible to anyone of any age, And people can learn billions of things in a lifetime faster than before. Also, The only reason the child in question isn't "mentally mature" is because they were taught not to be. I believe that if we really wanted too, Millions of children will be able to have the same knowledge like any other adult.
A well-educated 6 year old is capable of making basic adult decisions. Most 6 year olds are not well-educated, Because our education system is incredibly flawed. Our culture defines kids as kids, And generally speaking they are considered as fragile goods. Too inexperienced to have the capacity and perspective to understand how to make decisions not based on hormonal pressures. But society can change, And we can in theory have and let 6 year olds make adult decisions. Sex is like poison. You take the poison, Then you blame the manufacturers that you got sick. Consent is much the same. Think of like a person who's trying spicy food. He doesn't know it's THAT spicy, Yet he tries it anyways, Then he suffered the conciencinces.