Yes, you will have to leave unless your parent’s (including you) get their Green card before your turn 21.
This is normally known as ‘Ageing out’ in immigration.
You can change your status to some other visa type (like F1 for study, B1/B2 as visitor), but, the crux of the matter is that you are on your OWN once you turn 21.
As per US immigration law, you ceases to be dependent of your parents after the age of 21.
At that age, if you want to get green card for yourself, you will have to follow the same process that your parent’s followed, when the first came to US for work!