Age, and weight really aren't connected. Each snake is individual, you can have five two year old snakes, and they are all going to be different weights, yet all five are perfectly healthy. That is like thinking all 25 year old humans should weigh the same...
Its kind of hard to tell if there weight is alright based on just the age. All ball pyhons are different. My 1yr old male is 670g. As long as he looks healthy and isn't skinny, I would say he is alright.
Weight and length are related, that would be a better way to judge it, but it's all relative and based on the appearance of the snake more than anything