If you are not able to apply for a green card because of your unlawful presence in the US, nothing will change when you turn 18 become an adult.  IN fact, it will be much more difficult for you to try to apply once you are 18 because all the unlawful presence time will count against you.  If you depart the US before you turn 18, then you can make a clean start and actually obtain a visa to the US if you are the beneficiary of an approved petition.  If you remain past your 18th birthday, under the current immmigration laws, there is no way for you to adjust your status to permanent residence within the US.