That doesn't really answer the question though.

If the actions of these advertising companies are harmful, then everyone should be protected from them, not just one demographic that is considered to be more vulnerable.

And if they're not harmful, then in what sense are kids "vulnerable" to them and need protecting?

That is the justification, yes. And it is impossible to tell whether there is a kid, an adult or a dog on my side of the tcp connection without mass internet surveillance which is the actual reason.