If one of the parent is a U.S. citizen,would the offspring under 18 yrs. of age would it be automatically a U.S.citizen ?

If one parent is a US citizen and the child enter the country before their 18 birthday.... Yes, that child is automatically a us citizen.... Regardless they file N600 after their 18 birthday. It only matters that the child enters the country before they turn 18!

If the child with the U.S citizen they must file N600 before child turns 18.

Not if they weren't born in the U.S. The answer is no.