Is it permissible to pray behind an imam who said that the legal age for a child to enter school is six years, and is this considered an acceptance of man-made laws that contravene the law of God?

It is permissible to pray behind the aforementioned Imam, for his words do not indicate his approval of man-made laws. And setting the age of six for school entry does not contradict the Sharia, for educating a child is an obligation upon the parents, whether in public schools or elsewhere. The prayer behind this Imam is valid.