How old do you have to be to elope in Pennsylvania, with a legal adult as your wittness?

I'm 17 and in a few months my boyfriend of a year will also be 17. His parents are very much against out relationship because they don't agree with the fact that we've had premarital sex. My parents are both fully aware of what we've done and although they don't like it, they both support my relationship. I understand that 17 is a young age to make decisions, but I'm a very passionate person and I can see myself being with him forever. His parents are very abusive, in many ways and so that's a lot of the motovation to get him out of the house he's in. I know they'd never sign for us to be eloped but he has an older sister that's 18 and I'm sure she'd sign for him. My parents are all set and ready to let him move in because he really is a great guy, we're just trying to figure everything out. Another twist to the situation is that my boyfriend is adopted. So is his sister, I don't know what that adds to the mix, but I'm sure it's bound to screw something else up, everything else is attempting to work against us. Thank God for my parents. Thanks for answers.