in preparation for a flaming...
My DS (age 9) has ADHD. With medication, the ADHD (primarily impulsive and hyperactive) is well controlled.
When he goes to football camp in the holidays or swim lessons, we don't tell them. We've found that when less experienced people know, they tend to either have low expectations of him, or tend to try and over-discipline him. Basically, they treat him in a way that doesn't help, plus he doesn't want to be judged for it - which he has experienced and not liked.
When spoprts coaches etc are told nothing, they treat him like everyone else (which is correct as on medication, he is like everyone else), and it works much better.
Obviously, school know. And understanding friends and family. But judgy people (including friends and family) we don't tell, as we know they will treat DS as 'broken' or 'damaged' which would devastate him. He can cope with them thinking he is just rather wild and silly, as they have done since way before any diagnosis.
I'm sure that many people will disagree, but this works for DS at the moment. He doesn't identify as having a 'disability', so we leave it to him to tell people as he chooses.