My ds was first dx with ADHD at the age of 5. He sorta knew. We renewed the discussion when I found some books at the library when he was 8.
"Mom, this is just like me."
"Yes, I know, the doctor said you have this."
"Mom, WHY DIDN'T YOU TELL ME BEFORE?? This explains everything!!"
When his dx changed, we definitely kept him informed. He gets frustrated by it and sometimes wishes he didn't have it, but we try to emphasize the positive aspects - his knowledge, his loyalty, his affection, his appreciation for beauty, his ability to re-interpret things and help us all see things in a new way.
If his condition interferes with life, we discuss that too and try to brainstorm ways to lessen its impact next time.