At what age do children start to have their own opinions?

I'd say children start forming their own opinions quite early, maybe around 4 or 5 years old. That's when they begin questioning things and expressing preferences, like choosing what clothes to wear or what games to play. I remember my little cousin in Hangzhou - at age 5 she'd insist on wearing mismatched socks every day, saying it made her happy. That said, their opinions are still quite simple at that stage and heavily influenced by parents. It's not until later, maybe 10 or 12, when they start developing more complex views based on their own reasoning and experiences.