Is being a tomboy really a sign of ADD?

Catherine just strikes me as an ordinary girl with three older brothers at home.

I didn't think also that ADD went away when you got older, but the Catherine we see in the novel is not like an adult with ADD. She doesn't talk incessantly, she's not normally restless, she can focus for long periods of time on a book, she can sit and listen to other people playing music, she has normal conversations with people rather than perpetually jumping from topic to topic.