Most will slow right down to unnoticable growth at around 4-8 years of age. I have had a six year old snake come here and put on more size so it all depends on its history.
Feed your snake an appropriate sized item twice per week for its first year of life, then a appropriate sized item once per week after it's a year old. This will ensure proper, but healthy growth and when its between 4 and 6 feet, its full grown. As for sexual maturity this also differs but most corns are sexually mature when they are over 40 inches long with good girth. Males earlier than females as females need good size for healthy breeding since they lay the clutch and do most the work.
bmm