>this codebase already has this logic

At first glance this looks like it might be the halting problem in disguise (instead of the general function of the logic, just ask if they both have logic that halts or doesn't halt). I think we would need to allow for false negatives to even be theoretically possible, so while identical text comparison would be easy enough, anything past that can quickly becomes complicated and you can probably infinitely expand the complexity by handling more and more edge cases (but never every edge case due to the underlying halting problem/undecidability of code).

My main issue with this argument is that we never applied it to any other age gate created in the past. Why? Maybe part of it was control, but also because we know parents will fail their kids, and there were cases bad enough in the past that society decided to step in and protect kids even when parents fail.

If we really embraced this logic, then should we look at returning to the laws from before the 'protect the children' push of the 20th century. Compare this to some countries where kids can go buy beer. I've read stories from people in less regulated countries who use to buy beer for their parents when they were underage, and nothing was stopping them from buying it from themselves if their parents allowed it (or failed to stop it). Even a concept like child labor, why should we regulate that out to companies to control instead of depending upon parents to parent? When you consider web access as a person having some sort of transaction with a company, it generalizes to a very similar position of if a parent or the government should monitor that relationship for harm.