The "hyperactive" part of ADHD is kind of a misnomer, or at least doesn't mean hyperactive in the way that most people understand the word. Think of it more as a tendency to hyperfocus on specific things while completely ignoring other things.
You clearly have never met me in person. I am like a walking, talking Jim Carrey character. I'm also well into adulthood. For some, the hyperactivity is what most people think, and it does not always die down in adulthood.
Ok fair enough, I should have said that it's not always visible hyperactivity.