First, a cat at 12 weeks of age is still a cat. But she is still in childhood. Human babies and children are still people. But we call them children. So this 12 week old cat is still a child, and we call "child cats" kittens, until they reach about 9 months. Between 9 and 12 months, they're adolescent cats... similar to teenagers. Technically (and legally), teenage humans are still "children"... but they're leaving childhood and are almost through growing. At 12 weeks, a kitten is still like a little child, and "approximately" equivalent to a 4 year old child! So, think of this baby as sort of a preschooler.
This is a little calculator that can help you to figure out your cat's approximate physiological age compared to a human being. It will give you a rough idea of how your cat compares to you or to another human being. Just enter your cat's age in months or years, and it will give you the approximate human equivalent age!