Why doesn't the sentence use the người classifier to make clear that it is about a specific boy? Or is cậu a classifier on its own?
The word cậu actually means uncle (specifically your mother's brothers) but it's also used a lot in the North of Vietnam to refer to a male. Cậu bé means a young male.
Actually, "cậu bé" is a specific noun itself. So we don't need to use any classifier for it.
I translated "cậu bé" as "the kid", It corrected me as "the lad", And I see here "the boy".
Can you clarify to me the differences between all of these ? What age range does "cậu bé" refers to ?
"Cậu bé" means "the boy" in English,
"the kid" means "đứa trẻ" in Vietnamese
"be" means "baby" and usually is referred to a person fo younger age (i would say below 10yo). When you add "cau" in front of "be" that what indicate it is a male. So "Cau be" means "the boy" not "the kid" and I think "the lad" is wrong too as you uusally say lad for someone who is older.