[novok]

I think many who have 'adult' ADHD probably had a form of it as a child, but had early interventions or other mitigating factors that made them not assessed as a child. Such as being poor, or being an ADHD inattentive type, or a girl (ADHD can show up differently in girls vs boys), or an inattentive 'twice exceptional' who is also highly intelligent, so they were well behaved and did 'good enough' in school that nobody really thought to give them a neuro-psychological assessment. Or their undiagnosed ADHD parents think their ADHD kid is normal, because it's close to their behavior set and thus never really thought of it being an issue.

[adhd_thraway]

I can tell you directly that ADHD adults always had an ADHD when they were children, as that's not something you develop, it's just more like a skin color or a body build. Our minds just work differently.

ADHD isn't necessarily "hyperactivity" neither, sometimes it doesn't show till later years or even ever. Thus there are forms such as ADD, and ADHD in girls/women manifest vastly differently.

It's genetics. Depending on the study ADHD is genetic condition with 70%+ (latest high figure I heard is 77%) chance of passing down the children and around 30% occurrence in the 2nd degree. It's also additive, meaning that if both parents have mild-moderate ADHD the child has chance to hit a neurology jackpot. Based on family story I can exactly tell who have it in my family and it spans quite long. So when for example someone smashes a window or falls and bruises often it's quite often "oh it's always been like this with boys in our family" which is another gate preventing proper diagnosis and management.