It's not transient, it's a neurodevelopmental disorder that changes the fundamental structure of the brain and how it functions (this is a bit of a simplification, but it's true enough for this context).

People who 'grow out' of having ADHD either:

1. Developed coping methods that lessened the impact of their symptoms, making them functional enough to not be diagnosable (ADHD is only diagnosed if it negatively impacts your ability to function in two or more of the domains of work, social life, and home/family life). Often people who do this are still negatively impacted by their condition, but their problems are invisible and go unnoticed.

2. Never had ADHD to begin with, and instead had one of the many other psychiatric or physical conditions that can impact executive functioning (e.g. depression, sleep disorders, anxiety, malnutrition, etc.).

(wow I used the word impact a lot in this paragraph)