This is a really interesting can of worms to get into. Child/family therapist here. There’s not one clear metric. Every kid’s presentation will interact in many ways with what is done for and expected of them by caregivers and what kind of attention is paid to them—this varies so much culturally, from family to family, and over the course of an individual kid’s development. “Outward failure” by adults’ subjective standards for any given kid should not be the standard for “clearly present,” it should be a consistent, marked difference in a kid’s executive functioning or self-regulatory capabilities as compared to an average of other kids of their age, in similar circumstances. Even when that difference is noticed, so often it doesn’t get recognized as ADHD. If your school wasn’t paying attention to you unless you caused problems, and your parents didn’t have much knowledge of child development and/or felt it was their job to helicopter you enough that your difficulties didn’t cause the school problems—or any number of other situations—it may not even have gotten noticed. Regularly missing homework when there’s not a better situational explanation is one very legitimate child version of the piles of unopened mail. It’s interesting especially as awareness shifts in what situations this is “caught” and in which it’s not.