[lutorm]

You are doing a selective comparison.

No one is going to mistake a four-year-old for a grandmother.

True. But plenty of people will mistake a 14, 15, 16, or 17-year old for an 18-year old.

The Viacom v. YouTube case proved that even expensive lawyers spending hours doing research can't manage to get even 99% accuracy.

Again true, but the odds of a randomly selected uploaded full-length movie being illegally uploaded is, I bet, significantly larger than the odds of a randomly selected explicit movie featuring people of illegal age. So the prior coming from what fraction of available content out there is illegal according to the two standards would likely lead to a larger false positive rate for pornography.

[Natsu]

Saying that the porn filters don't work well only damages the case for copyright filters further. If the filters don't work well when people want them to, why would they work any better when people start trying to undermine them?