I have 4 month olds who are over 250g, and are feeding on medium adult mice.. It's a myth that feeding rats is better. Just feed what they will eat, have a consistent schedule, and they will grow.

The OP's snake might not have had a good start, therefore the smaller size. As long as the snake looks healthy, is eating every single week, sheds regularly, and is gaining weight, there isn't an exact weight the snake should be at, at that age. I would recommend a fecal check by a vet though, just to rule out internal parasites. Other than that, I wouldn't worry about it. By that age, the snake should be on adult mice or fuzzy rats, so if he/she's not, might want to bump it up to the right size.