My adoption mom and dad have always favored their biological kids who are both younger then me. It has been 5-6 years and I'm still treated with disrespect what can I do to earn their respect?

Just like you said--you EARN their respect.

I sincerely doubt they love their biological children more than they love you. As an oldest child myself, I know how often it seemed my parents loved my siblings more, but that simply wasn't true.

Have you calmly asked them, at a time when there is no argument between you, what you can do to earn their respect? Have you told them, calmly, and again when you're not upset with each other, that you feel less loved?

Part of being a teenager is feeling like an outcast. We have ALL gone through that, adopted or not.

Your parents can't work with you to help make things better if you don't communicate with them.

It probably has nothing to do with your being adopted. Most older kids feel like their younger siblings are treated better or loved more. The reality is that younger kids need more attention then older ones.