Google puts together a probable age based on the profile they collect about you.

They 100% know they are advertising to some children, even if just by mistakenly mislabeling children as older in rare cases. That's impossible to prevent through probabilistic means, so by implementing such systems they have accepted such an outcome.

There is no need to explicitly tell them your age.

That's the problem.

They know based on usage patterns. They know what age groups watch which videos. They use it to target children.

But because they didn't explicitly collect it, they can skirt the law.

The intent of the law is being ignored for profit.

Tech has solved more difficult problems. Maybe they should do that instead of relying on the same old “yes I was born in 1900” age verifications that are prevalent today.