I understand that someone past Bar/Bat mitzvah is responsible for their halachic obligations, but that doesn’t mean they are mature..
That’s right. They might not be mature enough for marriage but they are still responsible to fulfill their halachic obligations. Exactly.
It also doesn’t mean they have the ability to stand up to peer pressure or get the right answers to their questions.
No one drops out from one day to the next. It’s a process. If one wants to do good Hashem helps, and the same is with the bad.
Little decisions add up to create the person who drops out or stay frum. That is precisely why 12/13 year old are esponsible for their actions.
Teens today are not given enough acknowledgement for being the people they are and basically looked upon as the mixed up, immature kids. While teens don’t automatically make heavy choices when they turn 13, like if they want to be frum or not, it is the age where they do MAKE choices that all adds up in the end to become who they are unless they make a conscious decision to change afterwards.