Philosopher, I don’t know the last time you hung out with 12 year old girls or 13 year old boys, but they are not mature (as a general rule). The situation today is different from when boys and girls married much younger. People matured faster due to life experience.
I understand that someone past Bar/Bat mitzvah is responsible for their halachic obligations, but that doesn’t mean they are mature. It also doesn’t mean they have the ability to stand up to peer pressure or get the right answers to their questions.
I think you are simplifying this.