What would change when they turn 18? Could they just go about their lives as usual?
What would be the consequences for whom? The parents cannot harbor a minor on the run. The minute in custody will have to deal with the bench warrant for running away even after turning eighteen.
Running away is never a good idea. It may sound glamorous but you will regret it. Stay where you are, finish school and face your difficulties. Nothing better than overcoming them. Running away means you lost. As to the legal consequences, you and the other child will be listed as runaways if you are under 18.