I see a lot of teenage boys lean as well who just don't eat much, ectomorphs. they don't even exercise and they eat mcdonalds and are still lean.
its not that hard to do at that age, not sure why its so hard to believe. go to a high school soccer game and every kid out there is under 8% cause they run them all week leading up to the games.
maybe you are thinking 5% and huge and muscular, benches 350 pounds etc. in that case yeah I agree with you. but there are thousands of 17 year olds who are that lean. i used to coach high school sports and they were plentiful