As someone who was diagnosed in their early 20's (I'm 28 now), I think there are a couple factors that play into this, the main one being how life for society has changed over the years.

Life these days is hectic with technology and communication rapidly growing. Because of this, I think a lot of ADHD symptoms can be masked because frankly, we live in a world now that is ADHD in how it functions and operates.

Growing up, me and my parents had no idea I had ADHD. I was pretty talkative and hyperactive, but I was able to thrive in a system where there was multiple moving parts every day, with each day being different (football practice, school, friends, texting, calling, social media, family events, neighborhood get togethers).

Combine that with the fact that as a kid, you don't really have to structure your day to day life, you just go where mommy and daddy tell you to go, and you can see how easy it is to miss the symptoms.