A child is a child. They may be very intelligent, wise, and even more rational than an adult, but they still lack the emotional, mental, and moral maturity that distinguishes them from an adult. This sense of maturity emerges around the age of 12 for girls and 13 for boys. That is why the Torah holds them responsible for their actions at that age.
The meaning of Bar and Bat is son and daughter, signifying son and daughter of the Mitzvot, that is, obligated to observe them. Because it is such a significant moment in a person's life, it is customary to celebrate it with a meal that includes Torah passages, always according to the Jewish calendar.
Both men and women spend the months leading up to their birthday preparing for the moment, studying the importance of the Mitzvot and delving deeper into the meaning of being Jewish.
Even if a person did nothing at their Bar or Bat Mitzvah, they are still responsible and obligated to observe the precepts of the Torah. It doesn't depend on doing anything, but on being of the required age.