As with all walks of life, you have decisions to make and consequences to those decisions. Cycling early WILL affect your overall potential in a positive manner. In your teens, your body is still developing. Under normal circumstances test will only increase the overall size of a muscle fiber in an adult male. In a teen male higher levels of test 'could' result in more new fibers being created as a part of natural growth. My concern with youth cranking is not that it will have profound effects on their physique but rather their ability and potential to make mature and responsible decisions about their training and cycling. I would be all for a doctor AND trainer guided cycle in a sixteen year old who shows potential. Some things that a doctor and trainer could help with are ensuring dosages aren't in excess, training and form is proper, training will not stunt growth (going too heavy), and, amongst other things, proper cycling protocol is followed (cycling, bridging and/or PCT). After all, a cycle in your teens should only be used to lay a solid foundation for further endeavors into BB'ing (or any sport for that matter); Not to get super huge, stupid quick.
Another issue to consider is money. Not many parents will fund a cycle and not many kids make enough to fund a proper cycle themselves.
99% of kids these days don't have the level of maturity required to drive let alone play with hormones in their bodies which will (notice WILL not MIGHT) have serious long term effects on their health.
Then again, 99% of people who are legally considered adults don't either...