That is 8 months old!? Lord, how much/wahat are you feeding daily? I'll be honest, that seems like VERY rapid growth rate for a turtle at 8 months, even a snapper. You dropping live mice in there or something?! Lol
I picked him up as a "yearling" from the web site I found him on, and when he got to me I knew that he was certainly not a year old (as I check back, it was more like 4 months ago, not 6) ! He was literally like 1" scl, but looked like a good specimen so I didn't complain.
I know I'm providing adequate space, filtration, and food (he gets a variety daily: reptomin, shrimp, occasionally beefheart or boiled chicken, occasional feeder fish). I thought his growth was correct. I always worry about overfeeding turtles, since it is such common practice with inexperienced keepers.