Okay, I'll jump in.
In the late 80's I worked very briefly for a company that kept worker's compensation files. One of the clients of this company was the Oakland A's. Big Mac and Canseco's files were at least 20 feet thick and Mac only joined the team in 1986. Looking at the size of the file my thought was, "he's done." Then came BALCO.
These substances have provided true medical benefit for many players, allowing them to continue earning a living when if they didn't have these substances, they would be broken bodies on the unemployment line.
The problem is that using these substances provides a competitive edge so that people who don't need them for injury recovery are at a disadvantage, so they need to be able to load up.
Then if that is the level of competition to which young athletes are going to be required to attain if they want to be professionals, then they need to be able to load up, because they won't make it in the pros if they have to wait to build the necessary physique. However, that's a moot point, because there is no way kids' athletics can afford complex testing mechanisms. So, your kid will only be able to compete at a high level if he or she is juiced, because the other kids will be juiced.
So, if you make it legal for pros, even for medicinal purposes, you basically have to make it legal for everyone.