I’m 23 and I was diagnosed last year. It’s not odd at all to have a late diagnosis specially for girls. The symptoms do seem to get worse after a certain age which I assume is because of the anxiety that comes with more responsibility or something. So there is nothing to worry about.
I set up my first visit claiming that I have ADHD but I’m not exactly what you’d expect from a person with ADHD so I had a really long diagnosis procedure and I think I can help you.
The first test that I did after the general checkbox texts and conversations with the psychologists was called IVA. In this test you are given headphones and sit in front of a monitor. Numbers 1 and 2 are displayed and said and every time you hear a certain number you have to click the mouse. I scored really high. But they were not convinced. So I did QEEG which I’m told that is a brain map. It shows the activity (or the lack of it) in you brain. It should be really helpful and precise. It can be really helpful to diagnose not just ADHD but many other disorders and deficits (depression, ODD, etc.). Anyway most people are certainly diagnosed with the standardized questioners and a few sessions with a pro.
Just keep in mind that the only way to do this is to see a professional. Self diagnosis is harmful because one, you might be wrong and two even if you are right there is nothing you can do about it and that’s really frustrating.