I had a 12" jd, he was about 7yrs old. Most of the jds i've owned maxed around 10" around 3-4 years old. Most large new world cichlids do most of their growing within the first 2 years. If you are worried something might be slowing it down try treating with epsom salt. Dissolved in water then soak his food in it and let it dry out. The magnesium in the epsom salt will kill most internal parasites, it doesn't hurt the fish since they only absorb a certain amount into their bady and process it back out relatively easily. Word of caution it does kill inverts also (crays and snails)