I think everyone has experienced the symptoms of ADHD to some degree. The difference is the severity of the symptoms and how much they interfere with your ability to get tasks done.

I experienced the symptoms all my life but didn't get formally diagnosed until I was in graduate school and (among other things) couldn't just sit still and read an article start to finish in one go like everyone else.

When I started taking medicine for it, I couldn't believe how...easy my daily life became. It fixed symptoms I didn't even know I had. I didn't realize until that point how much I couldn't focus while driving to and from school/work, how often I interrupted others while they were speaking, all kinds of stuff.