It is impossible to account for what Rupp or any athlete does as a youth; in theory, kids can dope as much as they want to because they are not tested. Rupp entered the testing pool quite early, in 2004 at age 18 and through out his career is possibly the most tested distance runner in the world. Testing does not guarantee an athlete is not doping, but when you are tested frequently, doping becomes much more difficult and risky. Also, in the early 2000s, there were a number of products popular with kids, in particular football players containing creatinine and the word testosterone that you could buy at GNC. However, based on the price of actual 'anabolic steroid' testosterone, I assume what the kid's were taking back then were supplements somehow capitalizing on the name testosterone.
That is the end of the Rupp conversation, for me, this is really about 2 athlete's Lagat and Wilson whose situation I have a better understanding of.