its hard to judge from age alone, as snakes all grow at different speeds. You could post a picture of your snake next to a cd or something for scale and then we could tell you accurately.
Do you know the 1 and half times the width of your snake at the fattest part rule? Basically you try and feed a prey item that is anywhere from one to one and a half times the width of your snake at its widest part. I'd also say that if one pinky isn't leaving much of a lump when its in the belly then it might be time for two.