> In a non-adversarial political context

An oxymoron.

> where we trust the government and the future ones

Has never and will never exist.

> we could enact a law that's good today, but will be exploited in the future.

Sure but that's how pretty much all legislation works.

> I'd love it if it wasn't legal. But that brings its own can of worms.

It's probably fine as long as you include the clause "knowingly and intentionally". That doesn't imply age verification or anything else, merely that you act on information that you have and are aware of (and that you not intentionally design systems to work around that).

Also note that I never said anything about underage users. My example was targeting based on estimated user age. So in that example the age is estimated and it is illegal to target anything based on the value. (Of course to avoid a very silly loophole you'd also need to disallow targeting based on verified age as well.)