The way I see it, there is no set age. The age you start should depend on when you hit a plateau in training and have the cognitive skills to understand the ramifications of what they're doing. Physically, this can't happen until you're at least in your 20's...Not to say that 20 is the cut-off...because you really shouldn't (can't?) hit a plateau in your training until you've been at it for quite some time.
And when you hit that first plateau, change your diet and training, and go at it again. Then when you hit another plateau, change shit up again, and try again. Then if you hit another plateau, you can try busting through it with a light cycle...Then take things from there.
The way I see it, this stuff shouldn't happen until mid-late 20's...for the serious strength trainers. Of course, there will be the fewq that want instant gratification and won't want to work hard, and will just skip over that stuff. For those people I'd say go for it no earlier than 20...just so the side effects don't have major long-term effects.