I would pick the 15-20 age and then a category/s that you think would show ads kids are most likely to visit (click). Since I wouldn't count on misclicks on politics or some random category.
That's my thinking when I had to pick these.
Side note** You sure you can show smowtion on proxies? Thought I read here or DP that they were dropping proxy sites and not paying out. Perhaps someone here can verify that. Never used them before.