If I surf porn sites and it says all models are 18 or older and some girls come up that are questionable in age, do I have something to worry about?

Downloading child pornography is illegal and the penalties are severe. Just because someone says the models are all of age, don't believe it or rely on it. If you think a model is underage, erase the image immediately and don't go back to that website. If you are pretty certain the model is underage, contact the police and report the website.