Personality development is intricately tied to both biological growth and maturity. Human cells, including those in the skin, muscles, bones, and eyes, regenerate every seven years. However, nerve cells, which are crucial to brain function, stop regenerating around the age of seven, at which point 90 percent of brain development is typically complete. This is why a person’s core personality traits stabilize by this age. If nerve cells were to regenerate beyond this point, it could lead to changes in a person’s behavior, making their personality unstable from day to day. The Prophet ﷺ said: "There are three whose actions are not recorded: a sleeper until they wake up, an insane person until they regain their sanity, and a child until they reach puberty." (Abu Dawood and At-Tirmidhi) Because a child’s nervous system is still developing, their personality isn’t fully stable until they reach puberty. This explains why children are not held accountable for their actions until their personality and judgment become stable. When a child matures, their nervous system stops changing, and they develop consistent behaviors and responsibility. This is a sign of Allah's wisdom and mercy. He has made it so that we are not responsible for our actions until our personalities are fully formed. The stability of our nervous system is a part of what allows us to make decisions with a clear sense of right and wrong.