This is not as easy a question as it may seem. If he is a "citizen by law," then turning 18 will not effect his eligibility for the N-600. But if he is acquiring his status under some type of eligiblity that is only allowed for children under 18, he will age-out.
If your case is not already resolved, you should hire an attorney to research the law and give you a formal legal opinion on this. Our firm does this routinely.