>Yes, I'm a teenager ,18 and under
So not actually a teenager but a minor is what they mean and use what I would call deceptive language around it. But why?....
It's an organization for hacking working with high schools and young people. They don't want small children enrolled, and they don't want older people.
"teenager 18 and under" is perfectly fine description for 13-18 or 7th to 12th grade.