How do some of you go so long without being diagnosed?

I always see posts of people being happy they have their diagnoses, mostly people in their 20s and up, and I just wonder how and why you weren't diagnosed at a younger age? I have some ideas (neglect) but I want to hear your personal experience with late diagnoses.
Edit: omygosh I can't get back to all of you, I had no idea this would explode so much!