I'm sort of on the fence with this issue.
On one side of it, yeah, there are very few places that allow pornography at all that don't think that 18 is an acceptable minimum age, best as I know. Morally speaking, I think the Western world has largely accepted that 18 far enough along in life to be able to make some "adult" decisions. To some degree, this is true; it's when you're expected to start making them. The difference between a 17 year old and an 18 year old can be as little as one day. You need to give kids some time to build up experience making these sorts of adult decisions before you let them start making decisions that they might come to regret later, more from the stigma of a porn shoot being discovered by somebody you know than the actual incident itself.
Basically, I feel like 18 year olds are still teenagers. The magical number in the US is 21. By then, you've had at least 3 years of doing stupid shit after highschool under your belt, and have gained some sense of the consequences. I think that anyone concerned for the models involved could probably check their moral burden at this number.
On the flip side, we let people who've just turned 18 make the decision to ship themselves off to shoot guns at other kids who've just turned 18. I mean, if we're setting that age as the one at which we recognize the right to choose to participate in mortal violence (I know this sounds sort of anti-war, but it's not, I'm just stressing the point), then I think that letting people choose to expose some or all of their flesh on camera isn't too big of a leap.
Anyways, if they're at least of legal age and they fit my individual aesthetic sense, I'm all in regardless. For my personal tastes, I've found that all the 'lees that seem to suffer the most are in the 25-35 range, and nothing else about tickling is as important to me as the suffering of the 'lee.