Parenting adopted kids

I've always wanted to adopt kids even after I had my own kids. Currently my kids are 19, 16, 9, 8, 5, and 2. The younger four are adopted.

My dilemma has been in dealing with the older two. They often make remarks about how they feel like I favor the adopted kids over them! I'm pretty sure that I do no such thing. I treat all the six kids as if they were my own. Naturally I am stricter with the older kids, but only because I expect them to be more responsible

What my teens don't seem to realize is that I have to use a different parenting style on them because they are teenagers, not because they are not adopted. Parenting teenagers is a totally different monster, even though they are generally good ones. What can I do or say to show them that it is only because the younger ones have different needs to theirs?