There is an army of models whose appearances are at the younger end of the legal spectrum. I am not sure you can say they look younger than they are because there is no one look for any age. Some people look younger than others.
If you are into models who look younger then Asian models especially some of the Japanese will suit you. Asians look younger at the same age compared to those of European decent. Also some of the Latinos look young, but if you know anything about the origins of those indigenous to the Americas you will know they originally came for Asia across to Alaska and then down all the way to the tip of S America.
Young lookers are the likes of the earlier shots of Little Lupe, she is a classic. The problem here is you can unintentionally get into bother. There are many of the models from teen funs who were promoted on many many sites including peachy. Then years later there were the rumours some were underage, the site owners fell out, referring sites did not get paid and the site finished.
There was one poor guy in Puerto Rica who was on trial for Little Lupe videos, it was only when Lupe Fuentes actually turned up in court he was acquitted. The guy was so lucky. The authorities are after prosecutions not for the truth.