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jno, a child's first twelve months is his first year, not his 0th year. He goes through his first week, his first month, his first six-months and then completes his first year, at which point his age becomes one - i.e. he has completed his first year. Any 0th year is a phantom.
But look, Old_Geezer might not have been talking about the fictitious year AD 0. For me to assume that may be doing him a great injustice by insulting his intelligence.
So there is no point in continuing this unless and until he replies.