How can a 12/13-year-old be considered an adult? What actually happens at the age of 12/13? Please answer in great detail. Thank you.
The age of 12 (for a girl) or 13 (for a boy) is when the body of the child starts to physically mature, and therefore the child at that point is mature enough to accept responsibility, and to be held accountable for his/her actions. Although it is measured by signs of physical maturity, this physical maturity also shows that he/she is developing and it is also tells us that intellectually he/she is also maturing, therefore at this point he/she is considered to have “daas”. Therefore it is at this time that he/she is considered responsible, and held accountable for his/her actions. Although at that point the child does have daas, and is held accountable for his actions, it is clear that he is still not fully developed, physically, emotionally and intellectually, this is possibly why midinei shomayim regarding certain aspects, a child is held responsible only from age 20 (punishable by kares). However from this point the child is considered having maturity.
To add to this, it may be a little bit deeper. When a child shows signs of physical maturity, at that point the child can reproduce and technically become a parent. When a person becomes a parent he is responsible for the care of the child. Therefore at that point the child has responsibility.