I’m assuming it’s because he refers to people under 13 years old as girls/boys, though realistically it can be used for any young adult and is sometimes used in the army to refer to soldiers under the position. Just depends on how you use the term ig.
Yeah, technically it can be used to refer to any male or person identifying as a male at most ages, but most people as an adult would rather be referred to as a man/woman, not boy/girl.