It's age-based because it's defined that kids just can't (legally) consent, so lack of consent can be determined.
You can't determine whenever characters on picture are consenting or not (or pretending they're not consenting but actually do). Especially if they're drawings that bear no resemblance to any real persons. So, while your suggestion is logically right by its nature, it just can't work in reality.
Well, unless laws mandate that every porn-looking picture out there must be accompanied with a signed written consent proof. But that's not really possible, too.
For adult porn the legal system doesn't really care if they are consenting or real. A special needs teacher called Jane Longhurst was murdered by a guy who enjoyed violent pornography, so now mere depictions of particularly dangerous (and soon non-consensual) sex are made illegal. You could literally have signed written consent proof but if it looks like rape then it lands you in legal trouble because it might turn you into a rapist murderer.
You can't determine whenever characters on picture are consenting or not (or pretending they're not consenting but actually do). Especially if they're drawings that bear no resemblance to any real persons. So, while your suggestion is logically right by its nature, it just can't work in reality.
Well, unless laws mandate that every porn-looking picture out there must be accompanied with a signed written consent proof. But that's not really possible, too.