No offense, now I am not a self-proclaimed snake "pioneer", but a rattlesnake that won't cross the the path that a rope is placed over is a wive's tale(spelled correctly). Or the renowned "hoop snake" that grabs it's own tail and starts rolling down hill is also another well-known wive's tale.
But the fact that Indigo's do not reach lengths of 18 feet(or anything near that) doesn't mean for a SECOND that snake's do not continue growing throughout their entire lives.
Fact is, snake's DO continue growing, just much, much slower after they reach a very substancial size. Even if it is measured in micrometer's,and not feet or inches, it is still growth nonetheless.
Of course there is a pre-dispositioned genetic limit with any individual's growth too(as you mentioned), but as the snake aproaches that point, it's growth slows TREMENDOUSLY.
An eight foot Indigo under good conditions that is from exceptionally large parent stock could easily reach 7'9" inches within...let's say 6 to 8 years, but the additional 3 inches might take the entire rest of it's life to attain. The point here is...growth is growth, even if it can't be measured in feet, or many inches as when the animal was much younger. Snake's do not just one day stop growing, it is simply something that isn't nearly as noticeable as it once was, simple as that.
Just didn't think the previous poster(Jorge)needed to be corrected, when what he said didn't need correcting is all.
cheers, ~Doug
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"a snake in the grass is a GOOD thing" 
my website -serpentinespecialties.webs.com