This is a good question and there are a few reasons that younger guys are warned about using gear. At the end of the day, everyone is entitled to make their own decisions, but IMO, even the best built 19 yr old should wait a couple years. Steroid use is all about optimizing your hormones to make muscle gains. At that age, your natural hormone levels are at their peak. They will never be this good again. This part of the younger male body is still in development. Once you take a shot or a tab of AAS, your body begins producing less natural hormone. Over a short time, your body stops producing it's own test. This might cause more complications for a younger guy, that still is developing, once he comes off. Hormones are a very fragile balance and getting "fucked" at a young age, may throw off natural production for the rest of a younger guy's life. This means loss of sexual function and all the other sides that go along with low test, depression, loss of muscle mass, lethargy, just to name a few. 

Also, long bones, such as the clavicle, which will determine shoulder width, continue to grow into your early 20's. Steroids can possibly close these plates prematurely.

On another note, lets take the age factor out of the conversation. I wouldn't suggest steroids to anyone that hasn't alrady developed a good muscle base. Steroids will help speed up the muscle gain process, only when you already have the propor training and diet plan to grow. I've seen too many guys, of all ages, go on and off gear, never making progress because they never learned how to be consisitent in their diet and training. Every single person should be able to make some decent gains without gear before going on. No excuses. I hear them all. "Im an ecto and can't grow". "I have a slow metabolism and can't get rid of fat". Yes, these people can! They just don't know how. If you have a good natural, lean base to build from, you will get so much more out of AAS. You get out what you put in. 

Remember that this sport takes decades to be good at. Focus on today.