In my experience... In the 'Wild', they grow quite slowly up until 5-6 years old when they would reach around 2-4lb depending if male or female. From there on, the females can put on around 1-1.5lbs a year and lengthen in its skeletal growth. So in reality, a female could make double figures or more by 10-12years old. When the skeletal growth slows and ends (around 12-13 years max, in my opinion) they will then fluctuate in weight each year via the girth (expand and contract) and some may/will build in stature and put on more weight (and also drop it) throughout their life....depending on food supply/nutrition, health, habitat etc. Remember, only a few grow into monsters...but the majority don't because they don't have the built in ability/potential to do so. Also, not all heavy barbel have a long skeletal length. 25-30 years old seems around their lifespan, but as you know, some will die a lot younger and others may live on to a very old age.