Symptoms become more obvious as you age, which can feel like it came on later. As part of an assessment they do go through your childhood - if you had trouble in school for example that's a classic ADHD symptom, but there's a lot more to consider. It's not always obvious and I've met plenty of people in their 30s, 40s, 50s who only just realized they had it this year and got assessed.
Glyphack's experience definitely mirrors my own and I just got diagnosed with ADHD this year and medication absolutely helps my focus.
If you're wondering if you have ADHD this is basically the pre-assessment GPs use in the UK (which then continues to triage and then a full assessment):
> if you had trouble in school for example that's a classic ADHD symptom
or if you excelled at some topics which you found interesting but did middlingly or poorly at others...
I watched the skepticism form then evaporate in my psychiatrist as I explained that I did relatively well in school overall... except for the three or four classes I thought were boring, and how I consistently procrastinated but managed to pull off decent work right before hand.
(I got diagnosed after 30, though apparently my elementary school wanted to screen me.)