Kids can’t consent as true consent requires someone to be properly informed of the consequences of the action. Lack of life experience thus prevents true consent by definition. Which is why legally parents make the decisions for them.
The only other time I’ve heard this nonsense is out of the mouths of pedophiles by the way. If that’s not how you identify, or care to be identified, I suggest you educate yourself on consent and seriously question whatever information source you learned this nonsense from.