They all grow at different rates, kind of like people so you cant really assign a fixed weight to a length. You just need to look at the snakes overall condition. Does it look well rounded, the spine isn't sticking out or it doesn't have loose folds of skin hanging off of it? As for age, I am not aware of any way to just look at a snake and knows its age. Length really means nothing to age as it could have been powerfed or underfed or perfectly fed which would all yield different lengths at the same age plus snakes go through growth spurts as well. Best thing you can do is just remember the month/year you got her and go from there since you don't actually have the breeder to verify the date it was born.