I think most children start to have their own opinions when they are in primary school, maybe around six to ten years old. At this age, they begin to think for themselves and sometimes question what adults say. School also encourages them to speak up and share their ideas, so they slowly become more independent in their thinking.
ほとんどの子供たちは小学校、つまり6歳から10歳頃に自分の意見を持ち始めると思います。この年齢になると、自分で考え始め、大人の言うことに疑問を持つこともあります。学校も彼らに発言し、自分の考えを共有することを促すので、徐々に考える力が独立していきます。