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One of my kids was the youngest and started school aged four. One was the oldest, after repeating K. Both are successful. I can honestly say that neither of them ever brings up the issue and are fully independent adults.
Some younger students are very mature, some older ones are very immature. I can definitely say my kid who was the oldest in class did some stupid things as a teen, but my kid who was youngest was ready for school and was reading before the age of five.
There isn’t a one size fits all approach, but I do think countries such as Finland have it right with children starting formal education aged seven.