I've been lifting on and off since I was 12 - probably taken a total of 6-7 years off in that time (breaks ranging from a few months to a few years at a time), which means I've lifted around 14 years altogether. I was 125 lb when I started and about 215 lb at the end of that time. Put on about 25 lb in the last 4 - 5 years and I think I can still add another 20 lb or so. My arms went from 17 something inches to 19" during the last 3 years of that time.
Granted I'm at 15 - 20% body fat, but that still means you can make substantial gains 10 or even 15 years after you start lifting, as long as you are improving in terms of the amount of weight you lift (progressive overload), eat proper food and have decent genetics.
Judging what is possible by looking at the average lifter would make you conclude that it's not possible to reach a pro's level of development either. I bet not one of the guys who has said QFT or x2 to your post has trained for 20 years or more naturally, including yourself, to see what is possible
You think you can add 20lbs without increasing BF further? Or do you figure the muscle (hopefully) will stick once you diet down again? Well, who knows, you'll know in a couple of decades or whatever.

You're obviously not going to reach your potential in 2 years if you start training even before puberty.

Not even if you start at 15. I trained a few months when I was 9 and then started training regularly at 14-15 (was about 125lbs by the time I turned 15). I wouldn't have reached my full natural potential by 17 as I wouldn't even have been a fully mature adult, not even finished growing height wise, etc. But take a fully grown adult male, say a 20 year old, who can apply himself. He will put on the majority of muscle very quickly. Then he can bulk and cut, again and again, but it doesn't seem to change the size of the cut physique much.
Of course I haven't actually tried training naturally for 20 years. Too long an experiment to verify something not that important to me, something I could decide by observing others.

A friend of mine, a major juicer, once actually argued with me that a natural could reach Ronnie's size, if only he wouldn't run out of time. I don't think it works quite like that. I was really pissed he could make such a silly argument.
