You can't tell how old a guy is by looking at him. I've taught in middle schools, high (prep) schools, and universities. I remember two 13-year-old guys who looked old enough that they had no trouble buying alcohol in the states (where the drinking age is 21). I've also met juniors/seniors in university (likely 20 or 21) who looked five years younger.
Bear in mind also that mainstream media (i.e., Hollywood movies and TV) usually casts five or more years over the age of the character–characters who are supposed to be 18 are played by guys 23 or older; university students who are supposed to be 21 are usually played by actors 26-28 years old. So a "real" 18 year old will often appear to be a lot younger than he is.
Some studios (e.g., Cobra) claim that they have "accidentally" filmed underage models; others (e.g., GayTeensStudio, now defunct) did so repeatedly and were shut down. So there is indeed some material out there that will claim to be legal, but in fact has underage models. My suggestion is that if you have a film or scene with a model that you think is underage, just delete it. If you think it's a really flagrant situation, report it to a moderator, but don't post the name of the torrent in the forum...or you'll get people who actually like that sort of thing to download it, and the problem just gets worse.