Under the current immigration laws, you cannot get a green card within the US at any age if you entered the US unlawfully.  In fact, once you turn 18, you cannot even get it outside the US quickly because you start accruing unlawful presence time after you turn 18.  For you to qualify for an immigrant visa you will have to be petitioned by an qualified relative or employer and you will have to go back to your home country to consular process.  The problem is that once you depart the US, you trigger an automatic bar to reentry to the US if you have been in the US unlawfully after your 18th birthday.  If you have been unlawfully present for more than 6 months past your 18th birthday, you will be barred from reentry for 3 years; if you have been unlawfully present for more than 1 year after your 18th birthday, you will be barred from reentry for 10 years.