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):