During the early stages of cats, they actually need high levels of nutrients to make sure a proper growth. When they step into adult cat, they don’t need such high levels of nutrients any more which may lead to overweight.
The “one size fits all” pet food can’t suit the varied needs in different life stages of cats. Nowadays there are many pet food manufacturers providing all life stage foods for pets. They often just want to meet the laziness of the owners by claiming their products are perfectly balanced for each of these life stages.
So I wonder that those manufacturers may think cat’s diets can have little change during different life stages. The answer is obvious to us as the same effects on humans. When we are infant, we can only eat latex duct or milk as our digestion system are too weak to digest other food. As we grow up, the food varies a lot until we can eat as an adult. The human’s eating example effectively illustrates the same changes of diets in cats. So please be careful on your cat’s diet when she is already one-year-old.