Did you apply for DACA? If you don't have any status in the US, and if you did not receive DACA, then you began to accumulate unlawful presence beginning with your 18th birthday. If you depart the US before you accumulate 180 days of unlawful presence, then it appears that you will not need a waiver of an unlawful presence ground of inadmissibility. I don't know if you might have other grounds of inadmissibility. I suggest that you consult with an immigration attorney near you for details. Best wishes.
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The answer would depend on your entire immigration history and not just whether you accrued unlawful presence. As you may be aware under 18 you do not trigger unlawful presence but since turning 18 if you had any time unlawfully present then that would count toward unlawful presence. In terms of your full history there are other bars besides unlawful presence, which would need to be explored depending on what else has happened in your immigration history. The brief facts you give are not enough to determine whether or not you will need a waiver. Seek help from excellent counsel. Good luck
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If you only had accrued four months of unlawful presence, which started at your 18th birthday, you would not need a waiver for unlawful presence. Leaving the country for consular processing though, in my opinion, is one of those areas where you should always have a good lawyer. As the others have stated, you'll want to make sure that there are no other potential issues.