When you're young, it's very difficult to stay calm when you prove one of the unlucky teens to lose their hair without an acceptable explanation. Somehow, society forgives the young who lose their hair because of chemotherapy for cancer or as a result of some other natural accident. But any other hair loss marks you out as uncool. The really unlucky catch one of the fungal infections like ringworm which leaves swatches of hair on the ground as people walk around. This is deeply humiliating. Then you have the style conscious who put their hair under pressure by pulling it into position. After a while, pulling at the hair makes it fall out. But then come those who can be accused of having weak genes. Teens as young as 14 can be affected by male pattern baldness, according to findings from recent medical research.
When the FDA were reviewing the evidence from the medical trials, it was aware that male pattern baldness was possible among the younger teens, but it decided to limit the prescription license to those aged 18 and over. The decision by the FDA is always a balancing of benefits against risk. Young bodies are still going through the maturing process and the levels of different hormones can be changing quite quickly. To intervene in the balance of hormones in the young is dangerous. The FDA therefore erred on the side of caution. In part, this reflects the nature of the drug. It's not a cure for the genetic problem. It simply controls the process that would otherwise lead to hair loss. For the moment doctors are not supposed to prescribe Propecia to anyone under the age of 18, but if there had been a specific cure it is possible that the FDA may have reached a different conclusion.
So if this affects you, the choice is either to wait until you reach the magic age and put up with the inevitable problems of bullying, or to beat the prescription program by buying online. Since there can be longer term sexual problems, you have to want to stop the hair loss more than allow your sexual equipment to develop naturally. This should be a very difficult decision to take given the risk of impotence.