It's about impossible to guess the age by weight. It really depends on feeding size. Most pet shops and large breeders feed only what is needed. This helps them keep their cost down. Once you have the snake you are going to feed it what you think it should be eating. How the snake eats will determine its size, not the age.
Update...
Just switched her to live rat pups. She adjusted PERFECTLY! It amazing actually, the minute i put her.into her feeding tub, she gets ready to attck! She surely knows its dinner time! She.could care les that it was a rat.instead of a mouse! She is growing so fast. Here are a few pics I took last night.