Normal Cat Weight vs. Age
The Kitten
A cat is considered a kitten for the first year of her life. Kittens grow rapidly and are very active, and it's normal for her to look a bit lanky during rapid growth phases, as long as she is getting enough food. Regular checkups at the vet can help ensure that she is not underweight.
Adolescence
Most of your kitty's development is finished by adolescence. Between 6 months and 1 year, your cat will begin to be sexually active if you have not yet had her spayed. She is still very active, and so it's fine for her to be somewhat lean.
Adulthood
Things start to slow down for your cat as she enters adulthood at or after 1 year, because she's not growing or changing as quickly as when she was a kitten. It's important at this stage to be vigilant about checking that the numbers on the vet's scale don't start creeping too high.
Seniors
As cats age, they sometimes have health issues that extra weight can worsen, such as arthritis and heart, kidney or liver disease. Carrying less weight after the age of 12 years or so is important to Kitty's good health and ease of movement.