When you first bring home a fluffy little kitten, the notion of her ever growing up seems so far away and difficult to imagine. However, the reality is that kittens grow up fast. Expect your little one to begin going into heat before she hits her first birthday.
Female kittens often enter into their first heat cycle or "estrus" at around 5 to 6 months in age. The time frame varies, however, with some kittens entering into heat at as young as 4 months old, particularly from the Siamese breed. On the polar opposite side of things, many cats also don't reach full sexual maturity until almost a year in age -- especially Persian cats.
If you simply cannot tell whether your little one has gone into heat, it's not as difficult as you may think. The signs of the feline heat cycle are usually glaringly obvious. Your kitty may start behaving in an almost bizarrely affectionate manner, constantly rubbing her body against your legs or showing you her belly. She may start becoming a lot more vocal than before, loudly yowling and meowing the night away. She also may start behaving in a more irritable and aggressive manner than usual. You also may notice her positioning her body into a "mating" stance a lot, with her tail elevated into the air. Uncharacteristic restlessness and antsy behavior are also typical. Much to your annoyance, your kitten may also start spraying urine around your home in the least convenience places -- ugh.
When a queen cat goes into her heat cycle, it means for all intents and purposes, she is physically mature enough to mate and produce a litter of kittens. In general, felines are classified as being technical "kittens" until they reach about one year old. Even if your kitten may still be considered one herself, the possibility of her bearing offspring becomes a true reality with this new stage. This is especially true if your cat goes outdoors a lot and encounters unfixed tomcats.
If you're worried about your fluffy buddy going into heat, instead of waiting around for it, speak to her veterinarian about a suitable time for getting her spayed. Pediatric spaying is becoming more and more common, with many female kittens getting the surgical procedure done as young as 6 to 8 weeks. Not only does spaying prevent your kitten from going into heat, it prevents pregnancy and helps keep animal overpopulation under control -- not too shabby.