I work at a veterinary clinic. At this time of year we have a lot of people coming in with kittens. Most of them are either strays or from the local animal shelter, so people have no idea how old they are. The vet uses one pound for every month of age to give a rough estimate of age (so a 3 month old kitten should weigh around 3 pounds). It is just a guestimate, because even kittens in the same litter can vary by as much as a half pound in weight. We have even had one client that adopted littermates (one male and one female) and the female suprisingly was much heavier.