As someone who comes from IT, what are you talking about?
You rightfully assert that facial recognition and government ID are privacy invasive, but then you offer DNA verification as a less invasive tool? That's much more privacy invasive.
Also, it's impossible to determine someone's age from some DNA with the required accuracy. The law requires that 18+ content is available to someone who just turned 18 today and not available to someone who's 18th birthday is tomorrow. That's impossible to do from DNA, same as it's impossible to do from just facial recognition alone.
Carbon dating only works for dead materials since e.g. your skin or your skin is only ever roughly a month old and even blood cells only live for ~120 days. Also, again, carbon dating is not nearly accurate enough for the day-accuracy required.
The only day-accurate process that exists is verifying your identity against government ID. And here it hardly matters which kind of ID is used for that (facial ID/DNA/Fingerprint/...) since the issue at hand is the ID itself. Facial ID is by far the least privacy sensitive version of biometric DNA.
This process isn't great, no question about that, but the alternatives are worse if hard age verification is the goal.
That's why the whole goal is being called into question, since there's no non-privacy-invasive option to do real age verification.