Kittens are wee little bundles of joy, and it's often so hard to picture them as "grown-ups." No matter how much denial you're in, however, it's inevitable. Kittens grow up, and fast. As long as a queen cat is sexually mature, she can carry a litter of kittens.
A young cat is classified as being a "kitten" from birth up until roughly 1 year in age. Most female cats reach full sexual maturity at around 5 to 6 months, although it varies greatly. Some cats hit puberty as young as 4 months, while it can take over a year for others. Early puberty is especially common in breeds such as the Siamese, while others, such as the fluffy Himalayan, are comparatively late bloomers.
Female kittens who are mature enough to reproduce start going into heat cycles regularly. When a female goes into heat -- also called estrus -- it basically means she is ready to mate with tomcats. Unfixed female cats typically go into heat every few weeks or so. When a cat goes into heat, you will definitely know! The telltale indications include restlessness, pacing, uncharacteristically affectionate behavior, loud and piercing meowing, urine spraying and positioning her body into a crouching mating stance. If you observe any of these things, your cute kitten is surely old enough to have a litter of her own.
Male kittens become sexually mature at roughly the same time as their female counterparts -- around 5 or 6 months. Again, exceptions do occur. If a tomcat is ready to mate and father a litter, he will begin spraying behaviors. Boy cats spray their urine in order to notify nearby females they are indeed ready to mate. Also, if you notice that the kitten is significantly more vocal, it is because he is calling out to queens in the area.
If you are worried that your cat is old enough to have kittens -- especially if she goes outdoors a lot -- spare yourself the grief and get her spayed immediately. Speak with her veterinarian about her age, weight and height. Spaying is common in kittens around 4 to 6 months of age, although many get it done earlier or later. Not only will you not have to worry about nonstop yowling and cleaning up smelly and persistent cat urine, you won't be contributing to feline overpopulation either. If your cat is male, the same thing applies. Neutering is helpful to you, your cat's health and your community.