[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.