Both, but I lean heavily to age. In general, once they enter their second winter they are normally feeding as adults. Boys mature much faster than girls. Girls can still have heavy growth at three winters, some taper off at two. Boys are generally a year ahead of the girls. As far as weight goes, there seems to be something magical that happens to girls around 1000 grams, boys 800 where they go off food. All this of course assumes that the snake is healthy to begin with.
I have taken in older snakes that were underweight for their age. These guys will pound rats sometimes two mediums a week until they get to the weight they want to be, then they will taper off to something normal. With balls, the snake knows how big it wants to be. Last night, an average sized female that laid a 11 egg clutch 2 weeks ago decided she wanted to eat. She wanted two mediums and she got them (that is a lot of food for a ball). She will probably want 2 again next week. After that she will voluntarily go on a one medium a week schedule. Once breeding season rolls around again her feeding habits will change again.
I have never personally had a ball overeat. They seem to reach the size and weight they want to be, then stop eating. I have heard of, but never witnessed a ball that was obese. But, and it is a big but, I only have breeding animals.