I have a very similar story. Come from a stable loving family and initially did well. Only after leaving home did I come up against the wall of ADHD. On reflection, my parents and teachers were providing a huge amount of support. Outside of that supportive environment things fell apart: lost many jobs, couldn't keep up with basic life tasks. If I hadn't been so lucky I could've easily been homeless.

I finally got diagnosed at 30 and the doctor was similarly surprised.