I sat my boys down when they were 7 and 8. I asked them if they knew what "adoption" meant.

They said yes, "when you pick a baby or kid to be yours because you want to love them forever".

I thought that was beautiful. So I said..."one of you is adopted"- before I finished geting the words out my older son pointed at the younger and said, "It's him."

The young one jumped with glee shouting, "Is it me? Is it me?" He was so excited that we had chosen him. (and I am too)!   
We adopted him at birth and big brother is 11 months older!

Sometimes we find out things aren't exactly as we thought, sometimes it's even better! It is all in how you want to frame the new information.