THe problem with your size question is that even if you find the information, it is likely going to be wrong, or based on a small sample of turtles...
The environment you have the turtle in, food, and type of food will play more role in growth rate than the age will. Think about a fish...some may grow huge in the wild, but are limited when kept in captivity. I'm not saying that a res gets much bigger in the wild, just making mention that growth is affected by environment.
Most likely, your turtle is fine. You could have had a male and a female. The female will grow faster and larger.
It's my opinion that you don't need to worry about a growth rate that's slow...it's the fast rate that could be a problem if the turtle becomes obese and can't breathe well.