ADHD is a neurological disorder that is present at birth. The cause isn’t external, it’s genetic. The American Professional Society of ADHD and Related Disorders have compiled guidelines for diagnosing adults with ADHD. Not because it starts in adulthood, but because they weren’t diagnosed as children. By the time an undiagnosed ADHD child reaches adulthood, they’ve created coping mechanisms (that don’t usually work long term) that makes the test given for children obsolete.

I personally was diagnosed as a young child, when smartphones didn't exist and my parents were very strict about the screens that did (TV and computer games, which I was only allowed to watch/play educational ones anyway).