While most of the questions on the N-400 form focus on the past five years, there is one question at Part 7 that asks about any trips outside the United States in excess of 24 hours.
As such, I think you made an innocent mistake here. You can send a certified letter to USCIS explaining your mistake. Bring a copy of the letter and the certified receipt with you to the interview. When you appear at the interview, explain the mistake you made and offer the letter of explanation to the officer. Again, it seems to me that your mistake was quite innocent and the officer is likely to be reasonable in light of this.
One thing you need to consider is whether or not your absences from the US were for long periods of time such that you may be deemed to have abandoned your green card status, If you were away for near a year or more the the officer may inquire further. If, on the other hand, your absences were for short time periods, I don't think you have much to be worried about.
He is a link to a a couple of articles on this topic to provide further information:
Best of luck,
Peter J. Loughlin, Esq.
Goldman and Loughlin, PLLC
400 Corporate Sq., Suite 163
Naples, FL 34104
239-643-5529 - Free Consultation
Notice: The above response is for information purposes only and should not be construed as legal advice or legal opinion for anyone on any specific facts or circumstances.
Actually, Form N-400, Part 7, Question C refers to all absences since you became a lawful permanent resident. There is no 5-year cutoff for this question.
If you were an LPR at age 6, then you will need to report this absence. You can hire an attorney to remedy the situation and to accompany you to the interview to make sure everything goes smoothly.