I'd argue they don't have age in common either in the first years of school - at the beginning of formal schooling in the UK in September, the difference betwee

I'd argue they don't have age in common either in the first years of school - at the beginning of formal schooling in the UK in September, the difference between the youngest child in the class who might only just have had their 4th birthday in late August and the oldest who might have had their 5th birthday in early September just before starting school means that one is almost 25% older than the other. That's huge.