They start out being cats, they're just small cats. If you mean at what age they move from the youth category to the adult category, it would be after they have gone through the cat version of puberty, are fully fertile, and physically capable of producing and caring for their own small cats safely. On a slightly less facetious note I would have to say at about one year of age. A cat can breed successfully by then. People may not want their pets to breed at that age, but feral cats certainly do.

For dietary purposes, i.e. when kittens can start eating adult cat food, it's anywhere from 6-12 months. It depends, really, on who you ask. Assuming you're asking for dietary reasons, I do have a few ideas on the subject. Making the switch from kitten to adult cat food can depend on the quality of the kitty's nutrition from birth to six months of age. For example, someone brought me a very sick kitty who had been rejected by his mother and was starving to death. Because he was denied his mother's milk and was forced to wean himself on dry adult cat food, he missed out on a lot of nutrients that would have helped him grow. At about 2 months old, he was half the size of his litter mates. After he was brought to me, I kept him on kitten food until he caught up in size for his age, he was about a year and a half when I switched him to adult cat food. If the kitty has had proper nutrition growing up, then, imho, I would switch at around 9-10 months of age. It's best to make the switch over a period of about a month. Just start mixing adult food in with the kitten food, gradually increase week by week until they are eating 100% adult food. Kitten food is very high powered food with a whole lot of calories for the little guys to burn up. Once they've done most of their growing and their level of activity decreases, you want to switch to an adult maintenance type food so you don't end up with obese kitties. Here are some helpful links about this transition: http://www.iams.com/en_US/jhtmls/nutrition/sw_NutritionQuestions_qanswer.jhtml?li=en_US&sc=C&bc=I&pti=PN&qi=131 http://www.drsfostersmith.com/pic/article.cfm?dept_id=0&siteid=1&acatid=297&aid=471#answer%208 http://www.purina.co.uk/cat/article.asp?id=47 Hope this helps...

There is a table on this webiste that has cat years compared to human years. http://www.xmission.com/~emailbox/cat_age.htm Hope I Helped!

When they hit puberty at four or five months (according to a Nature show).

I THINK THERE ALWAYS A CAT!!!!!!!!!!!!!

9 months